UNIVERSITY  OF  CALIFORNIA 
AT  LOS  ANGELES 


A  COLLEGE  TEXT-BOOK 

ON 

QUANTITATIVE  ANALYSIS 


THE  MACMILLAN  COMPANY 

NKW  YORK   •    BOSTON   •    CHICAGO  -    DALLAS 
ATLANTA   •    SAN   FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON   •    BOMBAY  •    CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


A  COLLEGE  TEXT-BOOK 

ON 

QUANTITATIVE  ANALYSIS 


BY 

HERBERT  RAYMOND  MOODY,   S.B.   (M.I.T.),   A.M., 
PH.D.   (COLUMBIA) 

ASSOCIATE  PROFESSOR  OF  ANALYTICAL  AND  APPLIED  CHEMISTRY 
COLLEGE  OF  THE  CITY  OF  NEW  YORK 


THE  MACMILLAN  COMPANY 

1919 


COPYRIGHT,  1912, 
BY  THE  MACMILLAN  COMPANY. 


Set  up  and  electrotyped.    Published  September,  1912.     Reprinted 
June,  1914;  May,  1916;  February,  1917. 


NortoooS  Khfss: 
Berwick  &  Smith  Co.,  Norwood,  Mass.,  U.S.A. 


I'M 

IATI 


ACKNOWLEDGEMENT 
is   MADE 

TO   MY   WIFE 

EDNA  WADSWORTH  MOODY  (M.  I.  T.,  1893) 

WITH   WHOSE   EFFICIENT   COLLABORATION 
THIS   BOOK    HAS   BEEN    WRITTEN. 


I   AM    INDEBTED   TO 

PROFESSOR  LEO  FRANK  GUTTMANN,  PH.D.,  F.I.C.,  F.C.S., 

OF  QUEEN'S  UNIVERSITY,  KINGSTON,  ONTARIO, 

FROM  WHOM  I  RECEIVED  HELPFUL  SUGGESTIONS 

WHEN  ORIGINALLY  LAYING  DOWN  THIS  COURSE, 

FOR  CRITICISMS  AND  PROOF  READING. 


\N     THIS  book  is  issued  with  no  thought  of  enriching  the  literature 

*$\of  Quantitative  Analysis.      The  field  has  been  covered  so  often 
\    and   the   number   of   typical   analyses  suitable   for   elementary 

Q\  classes  is  so  limited  that  no  great  departure  from  regulation  courses 

j.    is  possible. 

The  aim  in  view  has  been  to  prepare  directions  which  shall  be 

**s  explicit,  clear,  adapted  to  those  to  whom  quantitative  analysis 
.    with  its  refinements  is  an  unknown  field  and  above  all  to  make 

£    obvious  the  unnecessary  pitfalls  that  consume  time.     For  the  last 

r  reason  the  author  has  placed  the  explanatory  facts  in  large  type 
on  the  same  page  and  directly  under  the  directions  for  procedure  to 
which  they  relate.  While  this  book  has  been  in  use  in  manuscript 
form  during  the  last  five  years  at  the  College  of  the  City  of  New 
York  this  has  been  effective  in  two  hoped  for  results:  (1)  The  pre- 
vention of  mistakes  that  are  so  often  made  before  the  student  has 
turned  to  the  end  to  read  the  explanations  and  warnings  in  the 
notes  and  (2)  a  clearer  understanding  of  the  reactions  and  changes 
at  each  step  of  the  work. 

Another  object  in  view  has  been  to  make  a  book  that  will  be 
useful  to  the  student  who  is  either  taking  up  quantitative  work  by 
himself  or  with  an  instructor  whose  classes  are  too  large  to  admit 
of  much  individual  attention.  This  is  more  often  the  case  than 
not.  It  is  the  writer's  experience  that  for  lack  of  specific  directions 
the  student,  while  the  instructor  is  otherwise  engaged,  makes  the 
fatal  error  which  necessitates  the  repetition  of  perhaps  hours  of 


Vi  FOREWORD 

work.  There  are  accepted  methods  of  manipulation,  the  result 
of  tried  experience,  that  insure  correct  analyses.  The  student 
cannot  be  expected  to  know  these  intuitively.  Too  much  inde- 
pendence of  action  at  this  stage  inevitably  leads  to  a  loss  of  time 
and  to  the  acquiring  of  disadvantageous  habits  of  manipulation. 
By  the  use  of  these  minute  directions  he  at  once  gets  into  the  habit 
of  correct  manipulation,  accomplishes  more  in  the  same  time  and, 
for  a  beginner,  gets  unusually  good  results. 

There  is  much  possible  latitude  in  the  choice  of  detail  of  methods 
in  the  analyses  selected,  but  standard  methods  which  have  been 
found  to  serve  satisfactorily  with  our  own  students  are  those  used. 

Only  such  facts  and  theories  as  are  necessary  to  the  full  under- 
standing of  the  development  of  the  subject  are  included.  For 
further  details  the  student  is  referred  to  his  instructor's  lectures 
and  to  larger  textbooks. 

The  author  will  gladly  confer  with  instructors  in  other  colleges 
concerning  his  system  of  "unknowns."  The  substances  used  for 
analyses  are  strictly  unknown  to  the  student  who  is  consequently 
forced  to  rely  upon  his  own  work.  A  qualitative  analysis  gives  no 
hint  as  to  which  sample  has  been  given  to  him. 

HERBERT  RAYMOND  MOODY. 

COLLEGE  OF  THE  CITY  OP  NEW  YORK. 


A   COLLEGE   TEXT-BOOK 

ON 

QUANTITATIVE   ANALYSIS 


QUANTITATIVE   ANALYSIS 


QUANTITATIVE  ANALYSIS  determines  the  amount  of  one  or  more 
of  the  constituents  of  a  substance.  The  methods  used  in  Quantita- 
tive Analysis  may  be  considered  as  refinements  of  those  already 
used  in  Qualitative  Analysis.*  When  it  is  merely  a  question  of 
determining  what  elements  are  present  in  the  substance  to  be 
analyzed,  these  less  precise  processes  of  Qualitative  Analysis  are 
adequate;  when  it  is  a  question,  however,  as  in  Quantitative 
Analysis,  of  determining  how  much  of  one  or  more  elements  is 
present,  the  reactions  selected  for  the  process  must  produce  more 
complete  separation.  To  isolate  completely  one  substance  from 
one  or  more  elements,  requires  definite,  standard  conditions  ful- 
filled with  the  most  scrupulous  care,  neatness  and  accuracy.  To 
illustrate: 

If  ammonium  oxalate  be  added  to  a  mixture  of  magnesium  and 
calcium  chlorids  to  determine  the  per  cent,  of  calcium  present, 
difficulties  are  met  by  adding  too  much  oxalate,  and  on  the  other 
hand  by  adding  too  little.  In  Qualitative  Analysis,  an  indefinite 
amount  of  the  ammonium  oxalate  added  to  the  mixture  of  mag- 
nesium and  calcium  chlorids  will  precipitate  most  of  the  calcium  as 
calcium  oxalate  and  leave  most  of  the  magnesium  in  the  filtrate. 
The  presence  of  calcium  is  shown,  which  is  all  that  is  required. 
In  Quantitative  Analysis,  there  are  two  facts  that  must  be  taken 
into  consideration.  First,  if  only  enough  oxalate  is  added  to  throw 

*  References  are  given  in  the  text  to  the  preliminary  experiments  in  "Chem- 
istry of  the  Metals"  which  apply  in  this  later  course. 

1 


2  QUANTITATIVE  ANALYSIS 

out  the  calcium  as  calcium  oxalate,  some  of  the  calcium  oxalate 
will  dissolve  in  the  magnesium  chlorid  and  make  the  per  cent,  of 
calcium  too  low.  Second,  if  enough  ammonium  oxalate  is  added 
to  convert  both  the  calcium  and  magnesium  to  oxalates,  then  the 
calcium  oxalate  will  drag  down  with  it  some  of  the  magnesium 
oxalate  and  make  the  per  cent,  of  calcium  too  high.  Hence  the 
analyst  is  between  two  dangers:  (a)  he  may  lose  some  of  the  cal- 
cium because  of  the  solubility _of  the  oxalate  in  magnesium  chlorid 
or  (b)  he  may  add  to  the  precipitate  through  contamination. 

Later,  when  analyzing  for  calcium  and  magnesium,  the  student 
will  be  given  precautions  which  reduce  these  dangers  to  a  mini- 
mum. Absolute  adherence  to  directions  as  to  quantities,  time  and 
manipulation  is  necessary  if  the  student  is  to  get  accurate  results. 


A  Quantitative  Analysis  may  be  made  by  Gravimetric  or  Volu- 
metric methods: 

In  Gravimetric  Analysis,  which  includes  electrolytic  work,  meas- 
urement by  weight  is  the  important  factor;  in  Volumetric  Analy- 
sis, measurement  by  volumes.  Electrolytic  Analysis  involves  the 
isolation  of  an  element  by  use  of  the  electric  current. 


SECTION  I 


GRAVIMETRIC  ANALYSIS 


GRAVIMETRIC  ANALYSIS 


Apparatus 

THE  student  is  already  familiar  with  most  of  the  apparatus  used 
in  gravimetric  analysis,  such  as  beakers,  evaporating  dishes,  flasks, 
lamps,  stirring  rods,  filter  papers  and  funnels. 

The  following  details  should,  however,  be  noted: 


(1)  Stirring  rods  must  be  rounded  with  even  more  care  than 
in  qualitative  work  to  avoid  the  possibilities  of  spoiling  beakers  by 
scratching.  Precipitates  adhere  to  such  scratches  and  are  removed 
from  them  with  difficulty.  For  the  complete  removal  of  precipi- 
tates, stirring  rods  should  have  at  one  end  about  one  inch  of  black 
rubber  tubing,  fitted  closely  to  the  rod  and  just  covering  the  end. 
This  rubber  end  should  not  be  kept  standing  in  a  solution,  espe- 
cially while  the  solution  is  boiling.  It  should  be  reserved  for  the 
latter  part  of  the  filtration  when  the  last  of  the  precipitate  is 
being  removed  from  the  beaker.  Such  a  stirring  rod  is  known  as 
a  "policeman." 


(2)  Ordinary  filter  paper,  the  sort  used  in  Qualitative  Analysis, 
is  not  suitable  for  the  final  filtration  of  quantitative  precipitates. 
Such  paper  is  used  only  in  preliminary  filtrations,  to  cover  the 
funnel  when  it  is  placed  in  the  drying  oven  and  to  place  on  the 
filter-stand  under  beakers  during  the  period  of  filtration.  For  the 

5 


6  QUANTITATIVE  ANALYSIS 

final  filtration  a  special  "washed"  filter-paper  is  used.  Any  of 
the  various  grades  are  suitable.  The  Schleicher  and  Schull, 
No.  589,  11  cm.  in  diameter,  is  a  convenient  size  and  a  satisfactory 
quality.  Each  paper  has  a  uniform  ash  of  0.00017  grams.  Allow- 
ance in  the  final  weighing  may  therefore  be  made  and  this  weight 
of  ash  deducted  from  the  total  weight  of  the  substance  in  the 
crucible.  Such  papers  are  expensive  and  should  be  used  only  for 
final  filtrations.  It  is  advisable  to  keep  filter  papers  in  envelopes 
or  boxes  to  prevent  them  from  collecting  dust  or  other  ponderable 
matter. 


(3)  Funnels  for  this  work  should  be  of  the  so-called  Bohemian- 
glass  type,  with  sides  at  an  exact  angle  of  60°  and  with  a  long 
stem  which  is  ground  to  a  point.  A  funnel  of  this  shape  filters 
much  more  quickly  than  one  of  the  ordinary  kind  (see  fact  7, 
page  36). 

In  addition  to  the  above  apparatus,  use  will  be  made  of:  (4) 
weighing  tubes;  (5)  wash  bottles;  (6)  casseroles;  (7)  filter  flasks; 
(8)  steam  baths;  (9)  air  baths;  (10)  crucibles;  (11)  triangles; 
(12)  desiccators  and  (13)  the  analytical  balance. 


(4)  Weighing  tubes  may  be  of  the  ground-glass-stopper  type 
with  either  a  flat  or  round  bottom  or  a  2f-inch  by  |-inch  test  tube 
fitted  with  a  good  grade  of  cork  stopper.  The  latter  is  equally 
desirable. 


(5)  Wash  bottles  used  in  Quantitative  Analysis  should  have  tips 
small  enough  to  emit  a  sufficiently  fine  stream  of  water  to  avoid 
spattering  precipitates  when  the  stream  strikes  them.  For  the 
convenient  handling  of  a  wash  bottle  containing  hot  water,  the 
neck  should  either  be  wound  with  cord,  covered  with  cork  sheets 
or  supplied  with  some  other  similar  device. 


(6)  Casseroles  are  more  often  used  than  evaporating  dishes, 
altho  they  serve  the  same  purpose.     It  is,  of  course,  fatal  to  an 


GRAVIMETRIC  ANALYSIS  7 

analysis  to  lose  even  a  minute  amount  of  the  solution.  The  handle 
on  the  casserole  makes  it  much  easier  to  avoid  spilling  and  is  more 
convenient  to  hold  while  filtering. 


(7)  Filter  flasks,  or  suction  bottles,  facilitate  the  filtering  of 
gelatinous  and  other  precipitates.  The  funnel  stem  is  inserted  in 
a  one-hole  rubber  stopper  fitted  to  the  neck  and  the  side-neck  is 
attached  to  the  vacuum  outlet  by  rubber  "pressure"  tubing.  The 
tip  of  the  stem  of  the  funnel  should  come  well  below  the  side  neck. 
In  order  to  avoid  the  possibility  of  the  introduction  of  liquids  or 
corrosive  vapors  into  the  vacuum  pipes  and  apparatus,  a  gas 
wash  bottle  must  be  interposed  between  the  filter  bottle  and  the 
vacuum  cock  on  the  desk.  To  avoid  breaking  the  tip  of  the  wet 
filter  paper  by  the  increased  pressure  on  it,  the  apex  of  the  funnel 
should  contain  either  a  perforated  platinum  cone  or  a  small  cone 
made  by  folding  a  "hardened"  filter  paper,  "S.  &  S."  No.  575, 
7  cm.  in  diameter. 


(8)  Steam  Baths  are  used  for  evaporations  which  are  rarely  made 
over  the  free  flame  because  of  the  possibility  of  loss  through  too 
violent  ebullition.  If  the  Bunsen  lamp  is  used,  the  beaker  or 
casserole  should  stand  either  on  a  sand  bath  (a  shallow  iron  tray 
containing  a  quarter  of  an  inch  of  sand)  or  on  an  iron  plate  covered 
with  a  sheet  of  asbestos  board. 


(9)  Air  Baths  are  used  for  drying  precipitates  in  funnels  and  for 
determining  water  of  crystallization,  hygroscopic  moisture,  etc. 
They  may  be  simple  copper  ovens  with  shelves,  heated  by  a  lamp, 
or  they  may  be  larger  and  heated  either  with  a  steam  jacket  or 
steam  pipes  in  the  base. 


(10)  Crucibles  for  all  ordinary  ignition  of  precipitates  may  be  of 
Berlin  porcelain  in  spite  of  the  fact  that  some  precipitates  are  not 
so  easily  brought  to  a  constant  weight  in  porcelain  as  in  platinum 
crucibles.  Such  porcelain  crucibles  should  be  heated  with  great 


8  QUANTITATIVE  ANALYSIS 

care  to  avoid  breaking.  However,  unless  expense  prohibits  their 
use,  platinum  crucibles  are  far  more  desirable  except  in  the  case 
of  precipitates  which  are  easily  reduced;  as,  for  example,  those 
obtained  in  lead  and  phosphorus  determinations.  In  such  cases 
the  reduced  metal  will  alloy  with  the  platinum  and  spoil  the  cru- 
cible. For  work  of  this  kind  porcelain  crucibles  always  should 
be  used. 

Two  important  facts  should  at  this  point  be  learned: 

(1)  Platinum  alloys  with  certain  metals.     A  student  sometimes 
forgets  this  until  he  has  ruined  his  expensive  platinum  crucible. 

(2)  Before  selecting  the  sort  of  crucible  for  use,  the  chemical 
change  that  is  to  take  place  in  the  precipitate  which  is  being  ignited 
should  be  considered. 

Platinum  containers  should  be  used  for  most  fusions  and  always 
for  the  treatment  of  substances  with  hydrofluoric  acid. 

Whichever  crucible  is  employed,  it  must  first  be  ignited  and 
cooled  in  the  desiccator  before  weighing.  Otherwise,  when  it  is 
ignited  with  the  precipitate,  the  crucible  itself  might  change  in 
weight. 

Crucible  covers  are  always  weighed  with  the  crucible,  as  ignited 
precipitates  should  be  weighed  covered  in  order  to  avoid  absorp- 
tion of  hygroscopic  moisture.  Covers,  by  keeping  out  the  air 
currents,  also  prevent  the  loss  of  finely  divided  precipitates  during 
ignition. 

Crucibles  must  be  cleaned  after  each  analysis.  Porcelain  cru- 
cibles may  be  cleaned  by  using  any  solvent  of  the  adhering  sub- 
stance which  does  not  react  with  porcelain. 

If  platinum  crucibles  are  used,  the  student  should  observe  the 
following  rules  and  precautions  issued  by  J.  Bishop  &  Co.,  Malvern, 
Pennsylvania. 


CARE  AND  USE  OF  PLATINUM  WARE* 

To  insure  long  and  satisfactory  service,  platinum  ware  should 
be  perfectly  clean  and  bright  before  each  operation  in  which  it  is 
employed,  and  to  that  end  it  is  advisable  to  carefully  clean,  dry 
and  polish  it  immediately  after  it  is  used. 

*  Courtesy  of  J.  Bishop  &  Co.  Platinum  Works,  Malvern,  Pa. 


GRAVIMETRIC  ANALYSIS  9 

The  cleaning  may  be  accomplished  by  boiling  in  dilute  hydro- 
chloric acid  or  by  immersing  in  fused  potassium  bisulphate 
for  a  few  minutes  and  removing  the  salt  by  means  of  boiling 
water. 

By  rubbing  the  surfaces  with  moist  talc  or  fine  sea  sand  (free 
from  sharp  or  angular  grains)  the  platinum  may  be  freed  from 
adhering  substances  and  polished  without  injury  or  appreciable 
loss  of  metal. 

After  polishing,  the  platinum  should  be  thoroughly  rinsed  in 
distilled  water  and  finally  ignited. 


PRECAUTIONS  TO  BE  OBSERVED 

In  making  ignitions  or  fusions  in  platinum  vessels  by  means  of 
the  Bunsen  burner,  the  upper  non-luminous  cone  only  should  be 
employed,  and  not  the  inner  cone,  nor  should  a  smoky  flame  be 
used,  as  the  action  of  a  flame  containing  free  carbon  will  result 
in  the  formation  of  a  carbide  of  platinum,  causing  the  metal  to 
become  brittle. 

Fusions  in  which  hydrates  of  sodium,  potassium,  barium  or 
lithium  are  used  should  not  be  performed  in  platinum  vessels, 
as  they  attack  the  platinum  at  high  temperatures.  Great  care 
should  be  observed  in  igniting  phosphates  in  platinum  crucibles, 
as  the  presence  of  reducing  substances,  such  as  the  charcoal  of  the 
burnt  filters,  may  cause  the  reduction  of  small  quantities  of  phos- 
phorus, which,  combining  with  the  platinum,  render  it  quite 
brittle. 

Compounds  of  silver,  lead,  tin,  bismuth,  arsenic  and  antimony 
should  not  be  ignited  in  platinum  vessels  as  the  reduction  of  metals 
having  low  melting  points  may  result  in  the  formation  of  alloys 
with  the  platinum. 

Evaporations  and  fusions  in  which  chlorin,  iodin  or  bromin 
are  set  free  should  not  be  performed  in  vessels  of  platinum. 


(11)  Triangles.  During  the  process  of  heating,  crucibles  are 
placed  upon  triangles  of  such  a  diameter  as  to  admit  of  the  crucible 
resting  vertically  or  on  its  side  at  an  angle  of  forty-five  degrees. 


10  QUANTITATIVE  ANALYSIS 

For  all  ordinary  purposes  "pipe-stem"  triangles  answer,  but  those 
made  of  heavy  platinum  wire  (gage  18,  or  heavier)  are  in  some 
respects  better.  These  latter  need  not  be  made  entirely  of  plati- 
num, but  a  triangle  of  platinum  wire  may  be  stretched  from  copper 
loops  at  the  center  of  the  sides  of  a  larger  copper  wire  triangle 
(gage  12,  or  over).  The  triangle  must  be  kept  scrupulously  clean. 
If  it  has  not  been  well  cleaned,  after  a  carelessly  regulated  fusion, 
for  instance,  adhering  matter  may  attach  itself  to  the  next  cruci- 
ble heated  upon  it  and  increase  its  weight. 


(12)  Desiccators,  as  the  name  indicates,  furnish  an  inclosed 
space  in  which  the  air  is  perfectly  dry  and  from  which,  therefore, 
no  moisture  can  be  deposited  on  crucibles  or  precipitates  to  add  to 
their  weight  as  they  cool.  There  are  various  types  of  desiccators, 
a  most  convenient  form  of  which  will  be  found  in  the  five-inch 
Scheibler  which  is  fitted  with  a  three-hole  porcelain  crucible  plate 
and  filled  in  the  lower  chamber  with  a  few  sticks  of  solid  potassium 
hydroxid.*  The  ground-glass  undersurface  of  the  run  of  the 
cover  should  be  lightly  covered  with  vaseline  or  with  a  mixture  of 
equal  parts  of  beeswax  and  paraffin  melted  together. 


(13)  The  Analytical  Balance  is  sufficiently  sensitive  to  weigh  to 
one  ten-thousandth  part  of  one  gram.  The  pillar,  beam  and  the 
pans  are  its  three  most  essential  parts.  The  pillar  is  surmounted 
by  the  beam,  the  halves  of  which  are  termed  the  "arms,"  two 
equal  divisions  of  a  lever.  The  beam  is  supported  on  a  fulcrum 
at  the  top  of  the  pillar.  The  arms  are  capable  of  adjustment  in 
length  by  means  of  a  micrometer  screw  at  the  ends.  Two  equal 
loads  on  the  two  pans,  therefore,  ^produce  equilibrium  or  equal 
oscillations  on  either  side  of  the  center.  The  right-hand  arm  is 
divided  into  five  or  ten  equal  parts,  each  in  turn  subdivided  into 
ten  equal  parts.  A  two-legged  piece  of  platinum  wire,  called  a 
"rider,"  may  be  set  down  upon  this  arm  at  any  point  by  means  of 
the  rider  hook,  which  is  moved  by  the  rider  rod  projecting  outside 
of  the  case  at  the  top  of  the  right-hand  side.  When  placed  upon  a 
whole  division,  the  rider  is  equivalent  to  a  corresponding  number 

*  Fused  anhydrous  calcium  chlorid  can  be  used  if  preferred. 


GRAVIMETRIC  ANALYSIS  11 

of  milligrams.  The  fractions  of  divisions,  consequently,  read  tenths 
of  milligrams.  For  accurate  results  the  balance  should  be  level 
on  the  table.  It  may  be  made  so  by  adjusting  the  height  of  the 
two  front  legs  of  the  case  until  the  spirit  level  or  plumb  bob  at 
the  base  of  the  pillar  indicates  proper  adjustment.*  Friction  in 
the  balance  at  the  point  of  support  must  as  far  as  possible  be 
avoided.  This  is  accomplished  by  using  "knife-edges"  to  give 
as  little  supporting  surface  as  possible  and  by  making  all  support- 
ing surfaces  of  hard  metal.  The  "knife-edges"  are  of  hard  steel 
resting  upon  agate,  f 

THE  WEIGHTS 

It  is  rarely  necessary  in  gravimetric  work  to  weigh  anything 
heavier  than  one  hundred  grams.  A  box  of  weights,  therefore, 
usually  contains  weights  of  the  following  denominations: 


50      grams 

20    grams 

10     grams 

10     grams 

5 

2 

1     gram 

1      gram 

0  .  5  gram 

0.2  gram 

0.1      " 

0.1 

it 

0.05     " 

0.02     " 

0.01     " 

0.01 

n 

a  total  of  99.99  grams. 

It  is  evident  that  if  the  weights  oxidize  or  corrode,  their  value 
would  change.  They  must  therefore  be  protected  from  dirt  and 
fumes  and  must  be  handled  only  with  forceps.  They  should  never 
be  left  out  of  the  box,  which  should  always  be  kept  tightly  covered. 
Pure  platinum  or  gold  would  be  the  most  permanent  materials  for 
weights  but  for  ordinary  work  their  value  prohibits  their  use. 
The  large  weights  are  of  brass,  tin,  etc.,  and  may  be  platinum  or 
gold  plated.  Fractional  weights  are  usually  of  pure  platinum. 

*  Such  adjustments  should  be  made  by.  the  instructor  and  not  by  the  beginner, 
whose  inexperience  is  likely  to  result  in  making  a  bad  matter  worse. 

t  For  details  of  construction  and  theory  of  the  balance,  the  student  is  referred 
to  Clowes  and  Coleman's  "Quantitative  Analysis,"  pages  1-14;  Morse's  "Quan- 
titative Chemistry,"  pages  1-30;  Fresenius's  "Quantitative  Analysis,"  pages 
1-26;  Olsen's  "Quantitative  Analysis,"  pages  7-20;  and  Treadwell's  "Analytical 
Chemistry,"  Vol.  II,  pages  6-16. 


12  QUANTITATIVE  ANALYSIS 


GRAVIMETRIC  ANALYSIS  is  the  process  of  determining  the 
relative  amount  of  a  constituent  of  a  substance  by: 

(I)  Weighing  a  definite  quantity  of  the   substance   to   be 
analyzed, 

(II)  Isolating  the  element  or  radical  sought  either  as  the  ele- 
ment itself  or  as  a  definite,  insoluble,  pure  compound, 

(HI)  Freeing  this  element  or  compound  from  admixed  liquids 
or  solids  by  filtering  and  washing, 

(IV)  Igniting  to  a  stable  compound  of  known  composition, 

(V)  Weighing  the  element  or  stable  compound,  and 

(VI)  Calculating  the  per  cent,  from  data  obtained. 


GRAVIMETRIC  ANALYSIS  13 


PROCESSES  EMPLOYED  IN  GRAVIMETRIC  ANALYSIS 

I.   Weighing 

The  weight  of  a  substance  taken  for  analysis  varies.  An  or- 
dinary charge  runs  from  0.2  gram  to  1  gram,  but,  under  some  con- 
ditions, 5  or  even  10  grams  might  be  required.  Within  certain 
limits  the  larger  the  amount  taken  for  analysis  the  more  accurate 
the  result,  but  no  amount  should  be  large  enough  to  give  an  over 
bulky  precipitate.  An  analyst  in  his  selection  is  guided  by  the 
quantity  of  the  unknown  compound  at  hand  and  also  by  the  re- 
lative amount  of  the  element  sought  in  the  substance.  If  it  con- 
tains a  large  amount  of  the  element  sought,  a  smaller  weight  of 
the  substance  is  taken  than  if  it  contains,  for  example,  only  a 
fraction  of  a  per  cent. 

A.  Precautions 

(1)  Always  use  the  same  balance. 

(2)  Sit  directly  in  front  of  the  balance  to  avoid  parallax. 

(3)  Borrow  neither  weights  nor  riders  from  another  balance. 

(4)  Allow  the  knife-edges  to  rest  upon  the  agate  bearings  only  when 

the  pans  are  swinging.  At  all  other  times  the  cradle  should' 
be  raised  by  the  knob  or  wheel  in  front.  Form  the  habit 
of  immediately  raising  the  cradle  the  moment  equilibrium 
has  been  established. 

(5)  Never  remove  weights  from  the  pan  nor  put  them  upon  it 

unless  the  beam  has  been  arrested  by  raising  the  cradle. 

(6)  Never  weigh  a  substance  upon  the  balance  pan,  but  have  it  in 

a  crucible,  weighing  tube  or  on  a  tared  watch  glass. 

(7)  Never  put  a  hot  nor  a  wet  dish  on  the  balance  pan  for  it  may 

both  hurt  the  surface  of  the  pan  and  change  the  weight.  A 
hot  dish  produces  ascending  air  currents  and  therefore 
weighs  too  little.  A  dish  below  the  temperature  of  the 
balance  produces  descending  currents  and  therefore  weighs 
too  much. 


14  QUANTITATIVE  ANALYSIS 

(8)  Keep  the  front  door  of  the  balance  closed  except  during  the 

time  of  putting  on  or  taking  off  the  weights  or  dishes.  If 
the  balance  is  provided  with  side  doors,  the  front  door  need 
scarcely  ever  be  opened.  The  final  operation  of  the  weigh- 
ing with  the  "rider"  should  be  done  with  the  case  entirely 
closed.  If  any  door  is  open,  air  currents  may  cause  an 
error. 

(9)  Weigh  all  precipitates  which  are  markedly  hygroscopic,  vol- 

atile or  absorbers  of  carbon  dioxid  with  the  crucible  cover 
on.  The  crucible  and  cover  should  therefore  be  weighed 
together  at  the  beginning  of  the  analysis. 

(10)  Never  handle  the  weights  except  with  the  forceps  provided 

in  the  box. 

(11)  The  center  of  the  pointer  scale  should  be  the  zero  point. 

Unless  the  balance  is  in  perfect  adjustment,  this  will  not 
be  the  case. 

Occasionally  determine  the  zero  point  as  follows: 
Imagine  the  divisions  on  the  pointer  scale  numbered  from  0  to 
20  from  left  to  right.     Make  three  readings  on  the  left  and  two 
on  the  right,  as,  for  example: 

2U50  l*'gOO        16-63  +  3.08  =  98 

3.00  16.25 


3  75  2)33  25          ^na^  *s'  ^ne  zero  P°mt> 

Q\Q  Og  Tv/r™  9.8,  is  0.2  of  a  space 

3)9,25  Mean,       16.63          at  the  left  of  the  mid- 
Mean,      3.08  die  division. 

The  position  of  the  zero  point  changes  because  of  varying  temper- 
ature, defective  condition  of  the  knife-edges  or  from  jarring. 

B.  Method  of  Weighing  . 

Unless  special  directions  are  given  for  weighing  in  a  tared  watch 
glass  or  crucible,  the  method  of  "direct  weighing"  of  the  charge 
is  not  to  be  used.  The  method  of  "indirect  weighing,"  weighing 
by  difference,  is  as  follows: 

The  weighing  tube  is  partially  filled  with  the  prepared  substance 
stoppered,  carefully  wiped,  weighed  and  the  weight  recorded. 
The  approximate  amount  of  salt  is  emptied  from  the  tube  into  a 
beaker,  the  stopper  replaced  and  a  second  weighing  made.  The 


GRAVIMETRIC  ANALYSIS  15 

difference  between  the  two  weights  is  the  weight  of  the  charge. 
This  process  is  given  in  detail  under  the  "Determination  of 
Aluminium." 

The  weight  of  the  charge  specified  in  the  directions  is  only  ap- 
proximate. For  example,  if  the  directions  call  for  0.5  gram,  a 
charge  weighing  0.4895  gram  or  one  weighing  0.5103  gram  fulfills 
the  conditions. 

C.  Directions  for  Weighing 

(1)  With  tongs,  take  the  article  to  be  weighed  and  put  it  through 

the  side  door  on  the  center  of  the  left-hand  pan. 

(2)  If  the  article  has  never  been  weighed  before,*  trials  should 

be  made,  according  to  (3),  to  determine  which  one  or  com- 
bination of  two  or  more  of  the  larger  weights  comes  the 
nearest  to  causing  equilibrium.  During  such  trials,  the 
cradle  should  be  lowered  but  part  way  and  the  pointer  carefully 
watched  in  order  to  avoid  the  violent  descent  of  either  pan, 
which  may  put  the  balance  out  of  adjustment. 

(3)  With  the  forceps,  put  the  large  weight  selected  on  the  center 

of  the  right-hand  pan.  If  the  right-hand  pan  descends,  the 
weight  is  too  large  and  the  next  smaller  denomination  should 
be  used.  If  this  proves  too  light,  add  smaller  weights  till 
the  total  weight  just  falls  short  of  balancing  the  article  on  the 
left  pan  and  finish  with  the  rider.  For  example:  A  twenty 
gram  weight  proves  too  heavy.  Raise  the  cradle.  Remove 
the  twenty-gram  weight.  Put  on  a  ten-gram  weight.  Partly 
lower  the  cradle  carefully.  This  is  too  small.  Raise  the 
cradle.  Try  a  combination  of  ten  and  five.  Lower  the 
cradle.  This  is  not  enough.  Raise  the  cradle.  Make  a 
combination  of  ten,  five  and  two.  Lower  the  cradle.  This 
is  not  enough.  Raise  the  cradle.  Add  one  gram.  Lower 
the  cradle.  This  causes  the  right-hand  pan  to  descend. 
Raise  the  cradle  and  put  the  one-gram  weight  back  into  its 
place  in  the  box.  Proceed  with  the  smaller  fractional 
weights  until  the  last  one  added  just  falls  short  of  causing 
equilibrium.  Close  the  door  and  determine  the  exact 
position  of  the  rider  necessary  to  produce  equilibrium. 

*  Forjthe  student's  guidance,  it  may  be  stated  that  an  average  porcelain  cru- 
cible and  cover  weighs  about  16  grams,  an  average  15  c.c.  platinum  crucible  and 
cover  about  12  grams  and  a  2J  inch-platinum  evaporating  dish  about  30  grams. 


16  QUANTITATIVE  ANALYSIS 

Experience  teaches  by  observing  the  swing  of  the  pointer,  about 
how  much  too  heavy  or  how  much  too  light  a  given  mass  is 
and  will  help  to  determine  the  next  combination. 

After  equilibrium  has  been  established: 

(4)  Raise  the  cradle. 

(5)  Add  up  the  weights  on  the  pan  and  record  in  the  notebook. 

This  should  be  done  in  ink  and  at  once.     This  rule  is 
insisted  upon.* 

(6)  Check  this  result  by  noting  the  empty  spaces  in  the  box. 

(7)  Remove  the  weights  with  the  forceps  and  put  each  in  its 

proper  place  in  the  box. 

(8)  Remove  the  crucible  or  other  container. 

(9)  Close  the  doors. 

(10)  Lift  the  rider. 

(11)  Call  the  Instructor's  attention  at  once  to  anything  that  may 

have  been  spilled  within  the  balance  or  to  any  part  of  the 
balance  that  may  have  become  disarranged. 


*  For  the  first  two  analyses,  all  recorded  weights  should  be  stamped  "checked  " 
by  the  Instructor  before  the  weights  are  removed  from  the  pan.  This  not  only 
insures  good  work  on  the  student's  part  but  corrects  errors  due  to  inexperience. 


GRAVIMETRIC  ANALYSIS  17 


n.  Isolating  the  Element  or  Radical  Sought 

SOLUTION  OF  THE  SUBSTANCE 

Getting  the  original  substance  into  solution  is  the  first  necessary 
step  in  the  process  leading  up  to  precipitation.  A  water  solution 
is  usually  preferable.  If  it  cannot  be  dissolved  in  water  it  should 
be  dissolved  in  acid.  Here  a  knowledge  of  solubilities  is  essential. 
For  instance,  an  alloy  or  compound  containing  lead  should  be 
dissolved  in  an  acid  like  nitric  acid.  An  unthinking  analyst 
might  try  sulfuric  acid,  not  recalling  the  fact  that  lead  sulfate 
is  insoluble  and  that  therefore  no  solution  could  result.  Many 
compounds  and  ores  are  wholly  or  in  part  insoluble  in  acids,  cold 
or  hot,  or  even  in  aqua  regia.  Such  substances  should  first  be 
made  to  undergo  a  preliminary  fusion  to  change  them  to  soluble 
compounds  (see  under  "Determination  of  Silica,"  page  78).  The 
principle  underlying  this  method  is  as  follows: 

All  carbonates  and  all  sodium  salts  are  decomposed  by  strong 
acids.  When  a  compound  is  heated  with  sodium  carbonate,  at 
the  temperature  of  fusion,  the  constituents  are  transposed  in  such 
a  manner  as  to  render  them  soluble,  carbonates  of  the  metals  of 
the  compound  are  formed  and  the  acid  radicals  combine  with  the 
sodium.  Since  all  carbonates  and  all  sodium  salts  are  decomposed 
by  strong  acids,  the  originally  insoluble  compounds  have  been 
changed  into  soluble  forms.  The  reactions  on  page  78  show  what 
happens  upon  the  decomposition  of  a  silicate  which  may  be  taken 
as  a  type  of  such  a  refractory  substance. 

REAGENTS 

After  getting  the  original  substance  into  solution,  the  charac- 
teristics of  reagents  selected  to  cause  precipitation  should  be 
considered. 

A  reagent  is  used  which  gives  a  definite  and  completely  insoluble 
compound  with  the  element  to  be  determined.  For  instance, 


18  QUANTITATIVE  ANALYSIS 

sulfuric  acid  could  not  be  employed  to  determine  calcium  altho 
it  gives  a  precipitate  of  calcium  sulfate  in  a  fairly  strong  solu- 
tion. Calcium  sulfate  is  somewhat  soluble  and  consequently  an 
oxalate  which  gives  a  completely  insoluble  compound  is  always 
used. 

Reagents  most  often  employed  are  acids,  their  sodium  or  am- 
monium salts,  or  alkalies.  As  they  are  readily  soluble  this  makes 
easy  the  preparation  of  their  solutions  for  reagents.  As  a  soluble 
salt  or  acid  results  from  the  metathetical  reaction  with  the  com- 
pound containing  the  element  to  be  precipitated,  most  of  it  passes 
into  the  filtrate  and  what  is  left  may  easily  be  washed  from  the 
precipitate. 

To  illustrate: 

Soluble  Precipitate         4 

FeCl3  +  3  NH40H  =  3  NH4C1  +  Fe(OH)3 
and 

Soluble         Precipitate 

BaCl2  +  H2S04  =  2  HC1  +  BaSO«. 

Other  compounds  than  the  soluble  salts  and  acids  used  for  pre- 
cipitations must  be  intelligently  selected  with  the  foregoing  facts 
in  mind. 

PRECIPITATION 

It  is  evident  that  to  completely  separate  one  element  from  one 
or  more  others  in  the  same  solution,  a  sufficient  amount  of  a 
reagent  must  be  added  to  precipitate  in  an  insoluble  form  all  of 
the  given  element,  which  may  then  be  filtered  off  from  the  simul- 
taneously occurring  substances  in  the  solution.  It  is  not  always 
possible  to  form  a  precipitate  entirely  free  from  other  compounds. 
In  such  cases,  subsequent  purification  is  resorted  to  (see  "  Deter- 
minations of  Calcium  and  of  Silica  "  for  two  types  of  such  proced- 
ure). When  working  with  substances  of  known  composition,  as  with 
alum  in  the  first  determination,  it  is  easy  to  calculate  the  amount 
of  reagent  required.  This  is  not  often  possible,  since  the  composi- 
tion of  the  substance  and  amount  of  each  element  present  is  un- 
known. Generally,  therefore,  the  reagent,  which  should  always 
be  hot  when  conditions  permit,  is  added  slowly  with  constant 
stirring  of  the  solution  until  no  further  precipitate  forms.  This 
point  may  be  determined  by  letting  the  precipitate  settle  and 
noting  whether  a  few  more  drops  of  the  reagent  produce  any 


GRAVIMETRIC  ANALYSIS  19 

precipitate  in  the  clear,  supernatant  liquid.  Filtrates  always 
should  be  re-tested  for  complete  precipitation. 

To  be  properly  done,  in  Quantitative  Analysis,  precipitation  re- 
quires a  good,  clear  knowledge  of  solubilities, — both  the  solubility 
of  the  substance  in  the  liquid  used  and  the  solvent  action  of  sub- 
stances simultaneously  formed  should  be  considered  (for  example, 
see  the  precautions  necessary  for  the  precipitation  of  calcium  in 
the  presence  of  magnesium,  pages  73  and  74). 

A  study  of  precipitation,  then,  the  process  of  making  insoluble 
compounds,  involves  knowledge  of  the  facts  underlying  the 
phenomena  of  solution. 

IMPORTANT  FACTS  ABOUT  SOLUTIONS* 

1.  Conditions  being  the  same,  a  great  variation  is  noticed  in  the 
solubility  of  different  solids  in  the  same  liquid. 

2.  No  solid  is  absolutely  insoluble.     Even  barium  sulfate  dis- 
solves to  the  extent  of  1  part  in  400,000  parts  of  water. 

3.  Heat  generally  aids  solution.     A  few  exceptions  to  this  rule 
are  noted.     Calcium  citrate  is  more  soluble  hi  cold  water  than  in 
hot  water.     Sodium  chlorid,  however,  is  about  as  soluble  in  cold 
as  in  hot  water.     Heat  is  an  aid  to  the  solution  of  metals  and 
minerals  in  acids. 

4.  Water  is  the  most  universal  solvent  and  has  some  solvent 
action  on  almost  everything.     As  previously  stated,  a  solvent 
must  be  selected  to  suit  the  individual  case.     No  general  rule  can 
be  given.     It  should  be  remembered  that  the  use  of  a  concentrated 
acid  often  appears  to  fail  to  give  solution  because  the  compounds 
formed  are  not  soluble  in  strong  acid.     Later  dilution  will  often 
cause  complete  solution.     Even  with  a  somewhat  soluble  com- 
pound, a  saturated  solution  may  result  and  more  water  be  needed 
to  destroy  the  saturated  condition. 

5.  Dissolved  substances  tend  to  distribute  themselves  evenly 
throughout  the  solvent  but  the  process  is  slow  and  should  be  aided 
by  stirring  or  shaking.     This  is  particularly  to  be  observed  in 
making  standard  solutions. 


*  See  "McPherson  and  Henderson"  and  other  larger  works  for  elaboration  of 
some  of  these  facts. 


20  QUANTITATIVE  ANALYSIS 

6.  All  substances  whose  solutions  conduct  the  electric  current 
dissociate  when  they  dissolve  in  water.      That  is,  the  molecule 
splits  into  two  or  more  parts  called  ions  which  are  as  free  to 
move  about  in  the  solution  as  are  independent  molecules.     These 
ions  carry  electrical  charges  and  hence  differ  in  their  properties 
from  the  atoms  or  molecules.     (See  under  "  lonization,"  page  85.) 

7.  The  equilibrium  between  substances  in  solution  may  be  dis- 
turbed and  a  reversible  reaction  may  go  to  completion  in  three 
ways: 

(a)  A  gas  may  form  and  escape  from  the  solution. 

(b)  An  insoluble  solid  may  form. 

(c)  Two  different  ions  may  form  undissociated  molecules. 

8.  Section  (b)  in  the  foregoing  paragraph  governs  the  phenom- 
ena of  precipitation  as  follows: 

If  HC1  and  AgNO3  are  brought  together  in  solution  the  following 
ions  will  be  present  H+,  Cl~,  Ag+,  NO^".  The  ions  Ag+  and  Cl~ 
will  then  set  up  the  equilibrium 


Silver  chlorid  is  almost  completely  insoluble  in  water  and  the 
formation  of  very  little  of  it  causes  a  supersaturated  solution  and 
the  excess  of  the  salt  precipitates.     More  ions  of  silver  and  chlorin 
then  unite  until  all  have  been  removed  from  solution. 
The  following  reaction  is  then  complete: 

AgN03  +  HC1  =  AgCl  +  HN03. 

The  table  on  page  21  will  give  the  student  a  knowledge  of 
solubilities  of  common  compounds. 


GRAVIMETRIC  ANALYSIS 


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22  QUANTITATIVE  ANALYSIS 


HI.  Filtering  and  Washing 

These  are  the  two  most  tedious  processes  of  Analytical  Chemis- 
try. Altho  it  is  possible  by  suitable  care  to  hasten  them,  they 
should  not  be  slighted.  They  are  facilitated  by  the  use  of  well- 
made  funnels,  accurately  fitted  filter  papers  and,  in  some  cases, 
by  the  use  of  suction.  At  best,  however,  they  are  both  time 
consuming  and  whenever  possible  should  be  carried  on  simulta- 
neously with  other  work. 

As  small  a  filter  paper  as  the  amount  of  precipitate  will  allow 
should  be  used.  To  wash  a  small  amount  of  precipitate  spread 
out  over  the  paper  requires  an  unnecessarily  large  amount  of 
water,  which  should  be  avoided.  The  paper  should  not  extend 
above  the  edge  of  the  funnel  and  should  preferably  come  only 
within  one-fourth  of  an  inch  of  its  top  edge. 

It  is  desirable  to  carry  on  as  many  filtrations  as  possible  at  the 
same  time.  A  number  of  these  will  not  require  much  more  time 
than  a  few.  Filtration  should  be  started  as  soon  as  possible  after 
the  beginning  of  the  laboratory  period  in  order  that  it  may  be 
finished  at  the  end.  It  should  be  kept  in  mind  that  washing  can- 
not be  finished  if  the  precipitate  has  been  allowed  to  stand  any 
length  of  time  in  an  incompletely  washed  state.  This  is  par- 
ticularly true  of  gelatinous  precipitates  such  as  the  hydroxids  of 
aluminium  and  iron,  which  dry  rather  quickly  and  form  a  non- 
porous,  film-like  mass.  Such  dried  precipitates  inclose  perma- 
nently any  soluble  salts  which  might  have  been  left  in  them. 
Unless  such  salts  happen  to  be  volatile  they  still  remain  after  the 
ignition  of  the  precipitate,  whose  weight  they  increase  and  cause 
a  consequent  error  in  the  result.  It  is  evident  that  it  is  just  as 
fatal  to  accurate  results  to  leave  in  the  mass  of  the  precipitate 
any  substance  which  should  be  washed  away  as  to  fail  to  obtain 
all  of  the  element  sought. 

The  precipitate,  then,  should  be  perfectly  washed  and,  in  order 
to  prove  that  it  is  washed,  the  last  few  drops  of  the  nitrate 


GRAVIMETRIC  ANALYSIS  23 

should  be  tested  for  some  ingredient  known  to  have  been  present. 
Strangely  enough  this  step  is  often  ignored  by  beginners.  Any 
constituent  known  to  have  been  in  the  filtrate  may  be  the  one 
tested  for.  If  an  acid  or  alkaline  solution  is  being  filtered  it 
is  customary  to  use  the  simple  litmus  paper  test.  This  is  not 
only  a  delicate  test  but  is  also  advantageous  in  that  the  litmus 
paper  can  be  washed  off  and  the  loss  of  the  tested  drop 
avoided. 

If  the  filtrate  is  to  be  used  for  the  analysis  of  some  other  sub- 
stance, only  the  fewest  possible  number  of  drops  should  be  used 
when  testing  for  complete  washing.  Such  tests  should  not  be  made 
until  it  is  fairly  certain  that  the  washing  is  complete  so  that  the 
loss  of  any  considerable  quantity  of  the  substance  in  the  filtrate 
may  be  avoided.  In  other  words,  the  filtrate  should  not  be  used 
for  testing  for  complete  washing  until  it  is  probable  that  the 
liquid  is  wash  water  only  or  an  extremely  dilute  solution. 

WASH  WATERS 

It  is  important  that  no  more  wash  water  be  used  than  is  abso- 
lutely necessary  to  remove  that  which  does  not  belong  to  the  pre- 
cipitate. When  the  filtration  is  sufficiently  advanced,  the  filtrate 
should  be  frequently  tested  both  to  save  time  and  to  avoid  un- 
necessary washing.  Too  much  washing  (1)  wastes  time,  (2) 
n  «dlessly  increases  the  volume  of  the  filtrate  and  (3)  as  no  sub- 
stance is  entirely  insoluble  if  treated  with  a  sufficient  volume  of 
water,  tends  to  dissolve  some  of  the  precipitate. 

Repeated  treatments  with  small  amounts  of  wash  water  give 
better  results  than  the  addition  of  all  the  water  at  once;  that  is, 
in  washing  a  precipitate  it  is  more  advantageous  to  wash  "by 
decantation"  ten  times  with  a  little  of  the  wash  water  each  time 
than  to  wash  once  only  with  the  total  amount  used  in  the  successive 
volumes. 

Hot  water  ordinarily  is  better  than  cold.  It  runs  through  the 
filter  more  quickly  and  is  more  efficacious  in  removing  the  sub- 
stance to  be  washed  out. 

In  order  either  to  avoid  the  dissolving  of  some  substance  in 
water  or  to  increase  the  solubility  of  the  substance  to  be  removed, 
special  wash  waters  are  often  used.  These  are  commonly  dilute 


24  QUANTITATIVE  ANALYSIS 

acids,  sometimes  dilute  alkali  and,  occasionally,  such  mixtures  as 
alcohol,  ammonia  and  water  or  acidified  solutions  of  salts  such 
as  ammonium  nitrate. 


EVAPORATION  OF  LIQUIDS 

Filtrates,  owing  to  the  volume  of  wash  water  used,  are  often 
much  too  dilute.  For  this  and  for  other  reasons,  such  as  the 
necessity  of  boiling  out  an  excess  of  volatile  alkali  or  acid,  it  is 
often  necessary  to  boil  down  liquids  which  later  are  to  be  treated 
quantitatively.  It  is  obvious  that  any  liquid  which  spatters  out 
removes  with  it  a  certain  amount  of  the  solids  in  solution,  a  loss 
that  later  causes  a  lowering  of  the  per  cent,  of  the  substance  deter- 
mined. Too  rapid  boiling  is  therefore  inadvisable. 

If  simply  the  evaporation  of  the  water  in  the  filtrate  is  the  object, 
the  beaker  should  be  covered  with  a  watch  glass  (sitting  upon  a 
glass  triangle)  and  should  be  allowed  to  evaporate  slowly  at  a  tem- 
perature just  below  that  of  ebullition.  If  a  solid  is  suspended  hi 
the  liquid  to  be  evaporated,  still  greater  care  should  be  exercised. 
Steam  forms  in  the  mass  of  the  solid  and,  upon  expanding,  forces  it 
violently  upwards,  giving  rise  to  what  is  known  as  "bumping," 
which  is  very  likely  to  cause  a  loss. 

Evaporations,  if  done  directly  above  the  lamp,  should  be  carried 
on  over  a  moderate  flame.  Preferably  they  should  be  done  on  a 
gas  or  electric  hot  plate  covered  with  a  thin  sheet  of  asbestos. 
Still  better  would  be  the  use  of  a  steam  hot  plate  or  steam  bath 
upon  which  there  is  no  possibility  of  ebullition  and  spattering  and 
which  may  be  left  running  all  night  without  danger  of  fire. 


GRAVIMETRIC  ANALYSIS  25 


IV.  Ignition  to  a  Stable  Compound 

Ignition  in  Analytical  Chemistry  is  the  process  of  heating  a  sub- 
stance without  allowing  the  direct  access  of  the  flame  to  the  com- 
pound heated.  It  is  accomplished  by  at  first  heating  the  crucible 
only  hot  enough  to  destroy  the  filter  paper  by  dry  distillation  and 
later  by  increasing  the  heat.  If  the  precipitate  is  to  be  subjected 
to  the  high  temperature  of  the  blast  lamp,  it  will  be  mentioned  in 
the  directions. 

Hastening  the  process  of  ignition,  which  is  a  temptation,  is  un- 
wise, as  it  is  almost  sure  to  make  the  result  too  low. 

Ignition  may  or  will  be  the  cause  of  any  or  all  of  the  following 
changes: 

(1)  Water  of  composition  or  water  mechanically  held  may  be 
evolved. 

(2)  The  filter  paper  will  distill. 

(3)  The  carbon  of  the  filter  paper  will  oxidize. 

(4)  The  composition  of  the  precipitate  may  undergo  a  chemical 
change,  as: 

CaC204  +  A  =  CaO  +  CO2  +  CO. 

(5)  The  precipitate  may  be  reduced  as: 

BaS04  +  2  C  =  BaS  +  2  CO2. 

(6)  The  precipitate  may  be  oxidized  as: 

BaS  +  202  =  BaS04. 

A  precipitate  may  or  may  not  be  changed  upon  ignition;  if 
changed,  it  must  change  completely  to  a  definite  new  compound. 
If  an  incomplete  or  indefinite  reaction  would  take  place  during  the 
burning  off  of  the  filter  paper  or  if  something  would  be  lost  by 
volatilization,  ignition  should  be  omitted  and  the  precipitate 
weighed  upon  a  "tared"  filter  paper  or  upon  an  asbestos  layer 
("felt")  previously  weighed  in  a  Gooch  crucible.  In  either  of 
these  latter  cases,  the  precipitate  on  the  paper  or  asbestos  should  be 
completely  dried  in  an  air  bath  and  weighed  without  ignition. 


26  QUANTITATIVE  ANALYSIS 


V.  Weighing  the  Element  or  Stable   Compound 

Repeated  ignition,  cooling  and  weighing  eventually  give  two 
nearly  the  same  consecutive  weights,  called  "constant  weights." 
Two  or  more  consecutive  weights  do  not  constitute  a  constant 
weight  if  there  is  a  greater  variation  than  three-tenths  of  a  milli- 
gram, 0.0003  gram.  The  completion  of  the  process  of  igniting  to 
a  stable  compound  is  shown  only  when  a  constant  weight  has  been 
reached. 

If  a  precipitate  is  hygroscopic,  it  is  necessary  to  hurry  the  weigh- 
ing that  the  water  absorbed  may  not  add  to  its  weight.  Before 
the  crucible  is  taken  from  the  desiccator  the  larger  units  of  the 
weight  of  the  crucible  may  be  placed  on  the  pan.  Upon  re- 
weighing,  all  of  the  weights  in  the  box  used  in  the  previous  weigh- 
ing may  be  first  put  into  place  and  the  weighing  finished  with  the 
rider. 


GRAVIMETRIC  ANALYSIS  27 


VI.  Calculating  the  Per  Cent,  from  Data  Obtained 

Sample  calculations  are  given  at  the  end  of  each  new  method  if 
there  is  any  new  principle  involved.  These  serve  only  as  a  basis 
upon  which  to  work.  Facility  in  calculation  may  be  acquired  by 
an  exhaustive  variety  of  problems  assigned  by  the  Instructor  or 
taken  from  the  following  books: 

Baskerville  and  Estabrook's  "Chemical  Problems." 

Well's  "  Textbook  of  Chemical  Arithmetic." 

Hale's  "  Calculations  of  Chemistry." 

Lupton's  "  Chemical  Arithmetic." 

The  initial  substance  is  weighed  only  to  the  fourth  decimal  place, 
as,  for  example,  0.2343  gram.  It  is  therefore  futile  in  the  various 
steps  of  the  calculation  to  use  figures  beyond  the  fourth  decimal 
place.  In  subtracting  the  ash,  for  instance,  use  the  figures  as 
follows: 

precipitate +ash  =  0.1389  or  ppt.  +ash  =  0.1389 

ash=  0.00007  ash  =  0.00014 

precipitate          =  0.13883,  use  0.1388    ppt.  =  0.13876,  use  0.1388. 

In  calculating  the  results  of  analyses  the  use  of  logarithms  is 
to  be  preferred,  not  only  because  it  saves  time  but  also  because  of 
the  fewer  figures  employed  there  is  much  less  possibility  of  error. 


FACTORS 

The  element  or  compound  isolated  is  not  always  weighed  in  the 
form  in  which  it  is  to  be  reported,  as,  for  example,  iron  may  be 
weighed  as  ferric  oxid,  Fe2O3,  and  reported  as  per  cent,  of  iron. 
The  relationshipjbetween  the  molecular  weight  of  the  element  or 

compound  sought  and  the  molecular  weight  of  the  "element  or 
Compound  reported  is  expressed  by  a  factor  found  by  making  a 
proportion  between  the  two.  The  factor  used  to  change  barium 


28  QUANTITATIVE  ANALYSIS 

sulfate,  BaSO4,  into  terms  of  barium  oxid,  BaO,  for  instance,  may 
be  found  as  follows: 

BaSO4 :  BaO  =  Weight  of  the  BaS04:  x. 
BaO 


BaS04 


X  Weight  of  BaSO4. 


Molecular  weight  of  BaO        153.4  . 

Molecular  weight  of  BaS04  "  233.5  "  JaC 

Therefore,  weight  BaO,  or  x  =  0.6571  X  weight  of  the  BaS04. 

A  table  containing  all  the  factors  and  their  logarithms  needed  in 
calculating  the  results  of  the  analyses  in  this  course  may  be  found 
on  page  155. 


GRAVIMETRIC  ANALYSIS  29 


PLANNING  THE  WORK,  ETC. 

The  student  should  particularly  note  that  the  one  who  accom- 
plishes the  most  analytical  work  is  he  who  plans  in  advance  and 
who  carries  on  several  processes  at  once.  He  reads  ahead  in  the 
directions  and  sees  the  place  at  which  a  wait  is  to  occur,  —  an 
evaporation,  a  dehydration,  etc.,  —  and  makes  sure  that  there  is 
some  other  work  sufficiently  advanced  to  at  once  occupy  the  time. 

It  is  necessary  to  state  that  absolute  cleanliness,  nothing  less, 
must  be  observed.  It  will  be  mere  luck  if  even  fair  results  are 
obtained  under  other  conditions.  A  linen  towel  should  be  used 
for  the  final  wiping  of  all  porcelain  and  glass  ware.  The  free  use 
of  labels  bearing  explicit  statements  is  necessary  if  one  is  to  avoid 
error.  Whenever  possible,  dishes  should  be  covered  with  watch 


30  QUANTITATIVE  ANALYSIS 


NOTEBOOKS 

Careless  records  of  results  are  almost  certain  to  bring  disaster. 
To  eliminate  this  source  of  trouble,  a  notebook  is  a  necessity. 

The  following  directions  are  to  be  observed: 

(1)  Use  the  left-hand  page  for  a  re'sume'  of  the  process  and  for 
keeping  a  complete  record  of  anything  unusual  that  happens 
throughout  the  entire  analysis. 

(2)  Record  weights  always  in  ink  directly  into  the  book.     Never 
use  loose  bits  of  paper. 

(3)  On   the   first   two   analyses  have   each   weight   stamped 
"CHECKED"  by  the  Instructor. 


GRAVIMETRIC  ANALYSIS 
(4)  Arrange  the  right-hand  page  as  follows: 


Determination  of . 


Analysis  begun:  date 
Analysis  ended:  date 


31 


Charge  1 

Weight  of  tube  +  substance  = 

Weight  of  tube  —  charge       =    

Weight  of  charge  = 

Weight  of  crucible  +  precipitate  +  ash  = 

II  U  ((  l(  (( 


Accepted  constant  weight  of  crucible 

+  precipitate  +  ash  = 

Weight  of  crucible  = 

Weight  of  precipitate  -f-  ash  = 
Weight  of  ash 
Weight  of  precipitate  = 


.  1st  heating, 
.  2nd  heating, 
.  3rd  heating, 
.etc. 


CALCULATION 

(See  sample  calculations) 

%of 


(5)  Make  all  the  calculations  in  the  book  and,  as  before  recom- 
mended, use  logarithms  for  calculations. 


32  QUANTITATIVE  ANALYSIS 


THE  FACTS  CONTAINED  UNDER  "APPARATUS" 
AND  "PROCESSES  EMPLOYED  IN  GRAVIMETRIC 
ANALYSIS"  MUST  HAVE  BEEN  LEARNED  BE- 
FORE WORK  IN  THE  LABORATORY  Is  BEGUN. 


BEFORE  BEGINNING  EACH  NEW  DETERMINA- 
TION, IT  Is  ESSENTIAL  FOR  THE  STUDENT  BE- 
FORE COMING  INTO  THE  LABORATORY  TO  HAVE 
STUDIED  AND  MASTERED  THE  PROCESS  AND 
ITS  ACCOMPANYING  NOTES. 


GRAVIMETRIC  ANALYSIS 


DETERMINATION  OF  ALUMINIUM 

IN 
POTASSIUM  ALUM,  KAl(SO4)2.i2H2O 


ALUMINIUM  is: 

(a)  precipitated  by  ammonium  hydroxid,  NH4OH,  as  alumin- 
ium hydroxid,  A1(OH)3,  in  the  presence  of  an  ammo- 
nium salt,  NH4C1; 

(6)  converted  by  ignition  to  and  weighed  as  aluminium  oxid, 
A1203; 

(c)  calculated  as  per  cent,  of  aluminium. 


REACTIONS 


I.  2  KA1(SO4)2  +  6  NH4OH  =  2  Al(OH), 

+  3(NH4)2SO.j, 

II.  (2A1(OH)3+A  =  2A1O(OH)+2H20) 
III.   ( 2  AIO(OH)  +  A  =  A1203  +  H20,          1 

or 
IV.     2  A1(OH)3  +  A  =  A12O3  +  3  H2O. 


34  QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  ALUMINIUM 

A  rigid  adherence  to  the  minute  directions  given  under  "Alumin- 
ium" will  save  time  and  will  give  skill  in  manipulation,  an  essential 
to  accurate  results. 

PROCEDURE1 

On  a  350  c.c.  beaker  (No.  3),  which  should  be  covered  with  a 

watch  glass,  place  a  label  marked  "Al." 
Wipe  off  the  cork  and  weighing  tube  containing  the  salt  and 

replace  the  cork. 
Weigh  the  tube  containing  the  salt  and  record  the  weight  on  the 

right-hand  page  of  the  notebook. 
Hold  the  weighing  tube  inclined  slightly  upwards  over  the  mouth 

of  the  beaker  and  carefully  remove  the  stopper,  which,  to 

prevent  loss,  is  held  over  the  dish.     Tip 

'  .  Weighing  the  charge 

the   mouth   of   the  tube  downwards  into 
the  beaker  and  with  slight  taps,  or  rotary  motion,  remove 
from  the  tube  an  amount  sufficient  to  make  a  charge  of 
about  1  gram2  of  the  salt.    Tap  down  into  the  tube  what- 


EXPLANATORY  FACTS 

1.  As  this  is  a  salt  of  known  composition,  each  student  should 
check  his  work  by  calculating  the  per  cent,  of  aluminium  in 
KA1(SO4)2.12  H2O.    This  calculation  must  be  put  in  full  on  the 
left-hand  page  of  the  notes. 

This  determination  is  not  done  in  duplicate. 

2.  AfeOs,   the  final   substance   weighed,   is  about  only   one- 
ninth  of  the  original  charge.     It  is  therefore  desirable  to  start 
with  a  somewhat  large  quantity. 


GRAVIMETRIC  ANALYSIS  35 

ever  remains.  Replace  the  cork  and  weigh  again.  At  all 
times,  in  order  to  avoid  moisture  from  the  hands,  hold  the 
tube  and  cork  as  lightly  as  possible. 

Record  the  weight  under  the  first  entry  and  subtract  to  get  the 
weight  of  the  salt  to  be  used  for  analysis. 

Dissolve  by  pouring  on  the  salt  100  c.c.  of  hot,  Dissolving  the  sub- 
distilled  water.  stance 


PROCEDURE 
Add  25  c.c.  of  a  solution  of  ammonium*  chlorid.3** 


EXPLANATORY  FACTS 

3.  Metallic  hydroxids  are  not  completely  precipitated  in  the 
presence  of  tartaric  and  citric  acids,  sugar,  etc.     In  such  cases, 
nonvolatile    organic   matter   must    be   decomposed    by   adding 
Na2COs  and  KNOs  to  the  solution,  evaporating  to  dryness,  fusing 
the  residue  and  extracting  it  with  dilute  hydrochloric  acid. 

4.  (a)  The  presence  of  the  ammonium  salts  (1)  insures  the 
complete  precipitation  of  the  aluminium  hydroxid,  Al(OH)s,  since 
such  salts  cause  the  reprecipitation  of  the  A1(OH)3  which  might 
be  dissolved  in  the  excess  of  the  reagent  and   (2)  affects  the 
physical  state  of  the  aluminium   hydroxid  by  preventing  the 
formation   of   a   colloidal  or  semisoluble    condition   (hydrosol). 
Boiling  alone  does  not  completely  change  hydrosol  into  insolu- 
ble hydrogel. 

(6)  It  is  usually  observed  that  precipitates  made  from  solutions 
containing  large  quantities  of  ammonia  salts  will,  when  washed 
with  hot  water,  become  sticky  and  wash  so  slowly  that  it  is  im- 
possible to  free  them  from  the  last  traces  of  the  salts  and  also  that 
very  perceptible  quantities  of  alumina  settle  out  from  such  wash- 
ings and  filtrates.  It  is  essential  for  successful  precipitations  to 
avoid  such  excesses. 

*  "Chemistry  of  the  Metals,"  Experiments  Nos.  217  and  218. 


36  QUANTITATIVE  ANALYSIS 

Add  perfectly  clear  ammonium  hydroxid  in  slight 

excess,5  that  is,  until  the  odor  persists  after 

the  solution  is  stirred. 

Cover  the  beaker  with  a  watch  glass  and  boil  gently  until  the 

odor  of  ammonia  has  nearly  disappeared  and  until  red 

litmus  paper  held  over  the  solution  is  only  slowly  turned 

blue.6 

Rinse  off  the  inside  of  the  watch  glass  into  the  beaker  by  means 

of  a  jet  of  hot  water  from  the  wash  bottle. 
While  the  precipitate  is  subsiding  — 

Fold  the  filter  paper  exactly  into  quarters. 

Open  the  paper  into  a  cone  and  force  it  into  the  apex  of  the 

funnel. 
To  complete  the  fitting  process  so  that  there  shall  be  no  air 

spaces7  between  the  glass  and  the  paper,  fill  the  funnel 

with  distilled  water,  let  some  of  it  run  through  the 

stem,  pour  out  the  rest  and,  where  it  is  necessary, 

press  the  paper  against  the  glass. 
Place  a  larger  beaker,  one  of  500  c.c.  capacity  (No.  4),  on 

a  clean  piece  of  filter  paper  on  the  base  of  the  filter 

stand. 
Arrange  the  funnel  so  that  its  tip  will  come 

just  below  the  curve  of  the  lip  of  the  ™£a0Jdn  of  the 

beaker  and  touch  its  side. 


EXPLANATORY   FACTS 

5.  Aluminium  hydroxid  is  soluble  in  an  excess  of  ammonium 
hydroxid.* 

6.  Upon  long  boiling,  by  decomposition  of  the  salts,  ammonia 
is  liberated  and  the  solution  becomes  acid. 

7.  If  an  air  space  is  left,  it  destroys  the  accelerating  effect  of 
the  weight  of  water  in  the  long-stemmed  funnel,  —  the  reason  for 
having  a  funnel  with  so  long  a  stem. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  217. 


GRAVIMETRIC  ANALYSIS  37 

Without  delay8  pour  off  the  supernatant  liquid  into  the  filter  as 
follows:  — 

To  avoid  spattering,  pour  down  a  stirring  rod  held  against 
the  lip  of  the  beaker,  so  that  the  stream  of  liquid  will 
strike  the  doubled  side  of  the  filter  paper,  not  upon  the 
surface  of  the  solution  hi  the  funnel.  The  level  of  the 
liquid  should  be  kept  an  eighth  of  an  inch  below  the  edge 
of  the  paper. 

The  entire  contents  of  the  funnel  should  be  allowed  to  run 
out  before  more  is  added.  In  the  long  run  this  saves 
time. 

The  mistake  should  not  be  made  of  transferring  at  the  start 
much,  if  any,  of  the  precipitate.     It  should  be  allowed  to 
settle  completely  in  the  beaker  and  the  supernatant  liquid, 
only,  be  decanted  off  through  the  filter  paper. 
Wash  the  precipitate  in  the  beaker  by  decanta-  Washing  a^  hy_ 

tion9    With    hot    Water.10       This    is    aCCOm-    droxid    by    decanta- 

plished  as  follows: 

Direct  carefully  down  the  rod  or  sides  of  the  beaker  a 
stream  of  hot,  distilled  water  from  the  wash  bottle 
into  the  precipitate.  Stir  with  a  rounded  glass  rod 
and  allow  the  precipitate  to  settle.  Pour  off  the  water 
through  the  filter. 
This  washing,  settling  and  filtering  should  be  done  three 

times. 
During  filtration  save  time  by  igniting  the  crucible  according 

to  the  directions  given  on  pages  39  and  40. 


EXPLANATORY  FACTS 

8.  Ammoniacal  liquids  exert  a  solvent  action  upon  glass. 

9.  By  decantation,  a  precipitate  is  washed  more  thoroughly 
and  in  less  time  than  if  transferred  at  once  to  the  filter  paper  and 
entirely  washed  in  the  funnel. 

10.  It  is  sometimes  advantageous  to  have  added  to  the  hot 
water  two  drops  of  ammonium  hydroxid  and  two  grams  of  am- 
monium nitrate  per  liter  (see  fact  4  (6)). 


38  QUANTITATIVE  ANALYSIS 

As  quickly  as  possible,11  transfer  the  precipitate  quantitatively 
to  the  filter  paper.    Hold  the  beaker  in  the  left  hand  using 
the  forefinger  to  keep  in  place  the  stirring  rod  which  should 
rest  across  the  top  of  the  beaker,  one  end  touching  and 
extending  beyond  the  lip.     With  the  right  forefinger  on  the 
movable  tip  of  the  wash  bottle,  direct  the  stream  of  water 
hi  such  a  manner  as  to  wash  the  precipitate  from  the  beaker 
down  the  stirring  rod  into  the  funnel.    When  the  precipitate 
is  apparently  removed,  rub  the  stirring  rod  clean  with  the 
"policeman"  or  with  a  closely  trimmed  goose  quill  and  hot 
water  from  the  wash  bottle.     Rub  the  entire  surface  of  the 
beaker  with  the  "policeman"  aided  by  a  fine  stream  of  hot 
water.     Pour  this  liquid  into  the  filter.     Finally,  closely 
scrutinize  the  beaker  held  hi  a  strong  light  to  be  sure  that 
it  is  perfectly  clean  on  the  inside.     Wash  off  into  the  filter 
any  precipitate  that  may  remain  on  the  "policeman." 
Wash  the  precipitate  hi  the  funnel  with  hot  water  as  follows:12 
Direct  the  stream  of  water  against  the  double  fold  of  the 
paper  and  then,  with  a  rotary  motion  of  the  wash- 
bottle  tip,  bring  the  jet  of  water  out  into  the  mass  of 
the  precipitate.    Let  drain  and  repeat,   washing  the  hy- 
Before  the  filtrate  is  tested  for  com-  drraid  'm  *• fmmel 
plete  washing,  the  upper  edges  of  the  paper  should  be 
washed  with  extreme  care.     Avoid  getting  any  of  the 
precipitate  above  the  paper  level. 


EXPLANATORY  FACTS 

11.  If  not  done  at  once,  the  precipitate  will  adhere  to  the 
beaker  and  be  removed  with  difficulty. 

12.  Aluminium  hydroxid  soon  hardens  and  cracks  and  there- 
fore should  at  once  be  washed  completely.     Filtration  once  begun 
must  be  finished  during  the  laboratory  period. 


GRAVIMETRIC  ANALYSIS  39 

Test  a  few  drops  of  the  filtrate  with  silver  nitrate*  solution  for 
chlorids.13  The  precipitate  must  be  washed  till  a  fresh  por- 
tion of  the  filtrate  gives  no  evidence  of  Testing  the  filtrate  for 
their  presence.  As  stated  above,  never  complete  washing 
completely  fill  the  filter  and  never  add  more  liquid  till  the 
filter  has  entirely  drained. 

Test  the  filtrate  with  a  little  more  ammonium  hydroxid  to  insure 
complete  precipitation. 

Cover  the  funnel  with  an  ordinary  11  cm.  filter  paper  wet  with 
distilled  water.  Stretch  it  over  the  top  by  pressing  the 
projecting  run  of  the  paper  firmly  around  the  outside  of  the 
funnel.  Tear  off  the  extra  paper  which  hangs  below  the 
edge.  Enough  will  remain  to  hold  it  on. 

Place  the  covered  funnel  in  an  air  bath  to  dry  the  precipitate. 
It  is  possible,  with  great  care,  to  ignite  the  precipitate  with- 
out  drying,   but  it  introduces  a  greater  Drying  the  pre- 
chance  of  error.     It  is  much  better  to  ciPitat® 
proceed  with  other  work  and  return  to  this  when  it  has 
dried. 

Wash  either  a  platinum  or  a  porcelain  crucible  thoroughly  and 
dry  with  an  "analytical"  towel  (pure  linen).  With  a  pair 
of  nickel  tongs,  place  the  crucible  upon  a  clean  pipestem 
triangle. 


EXPLANATORY  FACTS. 

13.  The  reaction  at  the  beginning  of  this  analysis  shows  the 
formation  of  K2S04  and  (NH^SO*.  These  and  also  the  added 
NHiCl  should  be  completely  removed  to  avoid  increasing  the 
weight  of  aluminium  hydroxid.  When  the  absence  of  chlorids 
is  assured,  it  may  be  assumed  that  the  two  sulfates  are  also 
removed. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  49. 


40  QUANTITATIVE  ANALYSIS 

For  a  few  minutes,  heat  the  partially  covered  crucible  to  redness 
with  a  properly  adjusted  Bunsen  flame.  Remove  the  Bun- 
sen.  After  the  redness  has  died  down,  with  the  tongs  put 
the  crucible  and  cover  in  to  the  desiccator  to  cool.  On  the 
porcelain  plate,  next  to  the  hole  holding  the  crucible,  there 
should  be  pasted  a  label  marked  "Al." 

After  twenty  minutes,  weigh  the  crucible  and   weighing  the 
cover,  replace  them  in  the  desiccator  and   crucible 
record  the  weight. 

When  the  precipitate  is  dry,  leave  the  cover  in  the  desiccator 
and  put  the  crucible  on  a  piece  of  glazed  paper.  Neither 
crucible  nor  cover  after  having  been  weighed  should  be  put 
anywhere  but  on  the  desiccator  plate,  the  triangle,  glazed 
paper  or  on  a  clean  watch  glass. 

Remove  the  filter  paper  from  the  funnel,  fold  it  carefully  and 
place  it  with  its  contents  in  the  base  of  the  crucible.  This 
is  not  the  type  of  precipitate  which  is  reduced  by  the 
carbon  of  the  filter  paper;  they  may,  therefore,  be  ignited 
together. 

If  any  of  the  precipitate  adheres  to  the  funnel,  wipe  it  off  on  a 
small  bit  torn  from  the  clean,  folded  portion  of  the  filter 
paper  and  put  it  into  the  crucible  with  the  main  precipitate. 

Place  the  crucible  at  an  angle  on  the  triangle  on  the  lamp  stand. 

If  the  dried  filter  paper  is  charred  with  great  care,  there  is  no 
danger  of  mechanical  loss.  If,  however,  for  any  reason,  the 
precipitate  is  wet  or  moist,  the  crucible  should  be  placed 
upright  and  the  utmost  pains  taken  not  to  heat  it  suffi- 
ciently to  cause  a  violent  escape  of  steam,  which  might 
drive  out  some  of  the  precipitate.  The  ring  of  the  stand 
should  be  so  adjusted  as  to  bring  the  crucible  above  but 
not  touching  the  flame.  When  the  paper  has  dried,  the 
crucible  may  be  turned  on  its  side. 

Too  much  care  cannot  be  exercised  at  this  point.  The  object 
is  to  destroy  the  filter  paper  by  dry  distillation  and  not  to 
burn  it.  Avoid  future  difficulty  by  implicitly  following 
these  directions.  It  is  possible  to  place  the  crucible  at 
such  a  distance  above  the  flame  as  to  accomplish  this  end. 
It  is,  however,  advisable  to  stand  at  the  work,  lamp  in 
hand,  with  a  low  flame,  ready  to  withdraw  it  if  the  escape 


GRAVIMETRIC  ANALYSIS  41 

of  volatile  matter  becomes  too  violent.     If,  through  the  in- 
experience of  the  student,  the  paper  takes   igniting  the 
fire,  the  tongs  should  be  at  hand  that  the   precipitate 
lid  may  be  taken  from  the  desiccator  and  the  crucible 
covered   to  extinguish  the  flame.     After    the    escape    of 
vapors  has   ceased  and  the  filter  paper  has   completely 
charred,  the  heat  may  be  increased  and  the  charred  paper 
ignited  till  the  carbon  is  oxidized. 

Heat  ten  minutes  over  the  blast  lamp.14 

Remove  the  crucible  to  the  desiccator,  the  cover  of  which  should 
be  put  on  with  a  slight  twist,  and  cool  until  the  crucible  has 
acquired  the  temperature  of  the  room.  A  red-hot  crucible 
should  not  be  placed  in  the  desiccator.  No  great  time, 
however,  must  elapse  between  the  removal  of  the  lamp  and 
the  insertion  of  the  crucible.  If  the  crucible  is  put  in  hot 
enough  to  expand  the  air  and  to  lift  the  cover  of  the  desic- 
cator, the  cover  should  be  carefully  readjusted.15  Three 
facts  should  be  borne  in  mind:  (1)  The  desiccator  always 
should  be  kept  covered,  except  at  the  moment  of  putting 
in  or  taking  out  the  crucible,  which,  after  ignition,  should 
never  be  handled  except  with  the  tongs.  (2)  The  cover 
of  the  desiccator  unless  inverted  should  never  be  put  down 
on  anything  but  the  desiccator  itself.  (3)  Before  weighing, 
the  desiccator  and  its  contents  should  be  put  either  in  the 
balance  room  or  in  a  place  in  which  it  will  acquire  the 
approximate  temperature  of  the  balance- 


EXPLANATORY  FACTS 

14.  The  final  ignition  in  the  blast  lamp  is  to  expel  the  last 
traces  of  water  and  insure  complete  change  to  oxid  (see  reactions 
II  and  III). 

15.  Carefully,  because  this  expansion  of  the  air  may  have  pro- 
duced a  reduced  pressure,  and  the  inrush  of  air  may  cause  some 
of  the  precipitate  to  fly  out  of  the  crucible. 


42  QUANTITATIVE  ANALYSIS 

Weigh16  and  record  Weights.  Weighing  the  ignited 

Repeat  heating  over  blast  lamp,  cooling  and   pfeciPitate 

weighing  till  the  weights  agree  within  three-tenths  of  a 
milligram.  This  will  bring  the  substance  to  a  "constant 
weight." 


EXPLANATORY  FACTS 

16.   Aluminium  oxid  is  hygroscopic  and  should,  therefore,  be 
weighed  quickly. 


GRAVIMETRIC  ANALYSIS  43 


SAMPLE   CALCULATION 

Weighing  tube  and  salt  =  6.0227  grams 
Weighing  tube  —  charge  =  4.9666  grams 
Weight  of  charge  =  1.0561  grams 

Weight  of  crucible  +  ash  +  A1203  =  11.9108  grams 
Weight  of  crucible  =  11.7973  grams 

Weight  of  ash  and  A12O3  =    0.1135  gram 

Weight  of  ash  =    0.0001  gram 

Weight  of  A12O3  =    0.1  134  gram 

Atomic  weight  of  Al  =  27.10 

"    0  =  16 

Molecular  weight  of  A12     =    54.20 
"  A1203  =  102.20 


The  ratio  of  A12  to  A12O3  =  =  0.53033. 


0.53033  is  the  constant  factor  that  represents  the  amount  of  Al 
in  A12O3. 

(Weight  of  A12O3)  0.1134  gram  multiplied  by  0.53033  (the  con- 
stant factor)  =  0.06013  gram  of  Al  in  0.1134  gram  of  A12O3. 

Since  there  is  0.06013  gram  of  Al  in  the  original  charge  of  1.0561 
of  the  salt,  then: 

0.06013  X  100      _  ftoc/  A1 
-L0561  -  =  5'69%AL 


44 


QUANTITATIVE  ANALYSIS 


CALCULATION  BY  LOGARITHMS 

The  logarithm  of  the  weight  of  A12C>3  minus  the  logarithm  of  the 
weight  of  the  original  charge  plus  the  logarithm  of  the  constant 
factor  of  the  amount  of  Al  in  A12O3  equals  the  logarithm  of  the 
per  cent,  of  Al. 


Log.  of  (weight  of  A12O3)  0.1134       =  9.05461-10 
Log.  of  (original  charge)  1.0561         =  0.02366 

9.03095-10 
Log.  of  (constant  factor)  0.53033      =  9.72454-10 

18.75549-20  or 
2.75549 

Anti-log,  of  2.75549  =  0.0569  or  5.69  %  Al. 


GRAVIMETRIC  ANALYSIS  45 


THE  DETERMINATION  OF  COPPER 

IN 
PURIFIED  COPPER  SULFATE,  CuSO4.5H2O  * 


Copper  is: 

(a)  precipitated  by  sodium  hydroxid,  NaOH,  as  cupric  hy- 

droxid  Cu(OH)2; 
(6)  converted  by  boiling  and  ignition  to  cupric  oxid,  CuO,  a 

type  of  precipitate  that  must  be  largely  removed  from 

the  filter  paper  before  ignition; 

(c)  weighed  as  cupric  oxid; 

(d)  calculated  as  per  cent,  of  copper. 


REACTIONS 

I.  CuSO4  +  2  NaOH  =  Cu(OH)2  +  Na«SO4. 
II.  Cu(OH)2  +  A  =  CuO  +  H20. 

III.  CuO  +  C  =  Cu  +  CO. 

IV.  3  Cu  +  8  HNO3  =  3  Cu(NO3)2  +  4  H2O  +  2  NO. 
V.  2  Cu(N03)2  +  A  =  2  CuO  +  4  N02  +  O2. 


*  If  this  course  is  to  be  limited  to  ninety  hours,  the  determination  of  copper  may 
be  omitted.  Otherwise,  the  value  of  the  preparation  of  a  pure  from  a  commercial 
salt  and  the  practice  of  the  separation  of  a  precipitate  from  the  filter  paper  before 
ignition  justify  the  time  expended. 


46  QUANTITATIVE  ANALYSIS 


PREPARATION  OF  THE  PURE  CuSO4.5H3O  FROM 
"  BLUE  VITRIOL  »  (COMMERCIAL) 


Weigh  50  grams  of  commercial  copper  sulfate  on  a  rough  balance. 

Dissolve  by  heating  in  150  c.c.  of  distilled  water. 

Add  1  c.c.  of  dilute  nitric  acid17  and  keep  the  solution  boiling 

gently  for  fifteen  minutes. 
Filter  and  put  the  beaker  containing  the  filtrate  in  cold  water. 

Stir  vigorously  to  get  finely  divided  crystals. 
Filter,  wash  the  crystals  with  a  small  amount  of  cold  water,  let 

drain  and  dry  by  pressing  gently 18  between  a  folded  sheet 

of  filter  paper. 
Put  the  crystals  on  a  clean  five-inch  watch  glass  and  let  dry  in 

the  open  air.     Turn  the  crystals  over  frequently  with  a 

clean   stirring   rod   until   they   no   longer  adhere  to  the 


Transfer  the  crystals  at  once  to  a  piece  of  glazed  paper  creased 
through  the  middle  and  empty  into  a  test  tube  that  has 
been  fitted  with  a  cork. 


EXPLANATORY  FACTS 

17.  Commercial  "blue  vitriol"  usually  contains  ferrous  sulfate. 
Ferrous  sulfate  and  copper  sulfate  tend  to  crystallize  together  and 
cannot,  therefore,  be  separated  by  "fractional  crystallization." 
The  ferrous  sulfate  is  consequently  oxidized  to  ferric  sulfate  by 
the  nitric  acid.     The  ferric  sulfate  does  not  tend  to  separate 
with  the  copper  sulfate  which  is  easily  obtained  in  crystals  after 
concentrating  the  liquid. 

18.  If  pressed  roughly,  fragments  of  the  paper  may  mix  with 
the  crystals. 


GRAVIMETRIC  ANALYSIS  47 


PROCEDUBE  * 


Fill  a   weighing  tube  two-thirds  full  of  the  purified  copper 

sulfate l9  and  fit  in  a  cork. 
Weigh  into  a  250  c.c.  casserole,  labeled  "Cu,"    „ 

,  '        Weighing  the  charge 

one   gram   of   the   salt    and    record    the 
weight. 
Dissolve  in   100  c.c.  of  hot,  distilled  water.   Dissolving  the 

The  solution   should   be  perfectly   clear.   substance 
Cover  the  beaker  and  heat  the  solution  to  boiling  f  (see  reac- 
tion II). 

While  stirring  with  a  glass  rod  having  rounded  Precipitation  of  the 
ends,  add,  drop  by  drop,  a  dilute  solution  of  hvdroxid 
sodium  hydroxid 20  (see  reaction  I)  hi  slight  excess.21 
Stir  constantly  and  continue  to  heat  for  ten  minutes.    Keep  the 
beaker  covered  when  possible. 

Test  for  excess  of  sodium  hydroxid  by  putting  a  drop  of  the 
solution  on  a  strip  of  red  litmus  paper  with  the  stirring  rod. 
Wash  off  the  litmus  paper  into  the  beaker  with  a  little  distilled 
water  from  the  wash  bottle. 


EXPLANATORY  FACTS 

19.  As  this  is  a  salt  of  known  composition,  each  student  must 
check    his    work    by  calculating    the    per    cent,   of    copper    in 
CuSO4.5H2O.    This  calculation  should  be  put  on  the  left-hand 
page  of  the  note-book. 

20.  When  cupric  hydroxid  is  first  formed  it  is  a  greenish  blue; 
it  turns  brown  at  once  at  a  boiling  temperature  and  then  nearly 
black.     This  indicates  the  change  to  cupric  oxid. 

21.  Cupric  hydroxid  is  insoluble  in  excess  of  fixed  alkalies  in 
dilute  solution. 

*  This  determination  is  not  done  in  duplicate. 

t  See  "Chemistry  of  the  Metals,"  Experiment  No.  102. 


48  QUANTITATIVE  ANALYSIS 

Rinse  off  the  underside  of  the  watch  glass  into  the  beaker  by 
means  of  a  jet  of  hot  water  from  the  wash  bottle. 

While  the  precipitate  is  settling,  fit  a  filter  paper  (S.  &  S.  "Ash- 
less,"  11  cm.)  hi  to  a  funnel  as  described  on  page  36.  To 
collect  the  filtrate,  use  a  500  c.c.  beaker  (No.  4). 

After  the  precipitate  has  settled,  pour  off  the  Filtration  of  the 
liquid  through  the  filter.  oxid 

Carefully  direct  a  stream  of  hot  water  from  the 

Washing  the  oxid 

wash  bottle  into  the  precipitate  and  stir. 

Boil  again.22 

Wash  by  decantation  several  times,  page  37. 

During  filtration,  start  the  heating,  cooling  and   Preparation  of  the 
weighing  of  the  porcelain  crucible,  page  39.    crucible 

Transfer  the  precipitate  quantitatively  to  the  filter  paper  as 
described  in  detail  under  "The  Determination  of  Alu- 
minium," page  38. 

Wash  the  precipitate  in  the  funnel  with  hot  water  until  a  few 
drops  of  the  filtrate  acidified  with  hydrochloric  add  give 
with  barium  chlorid  solution  no  test  for  sul-   Testing  ^  si^te 
fate.*     Direct  the  stream  downwards  onto   for  complete 
the  upper  part  of  the  filter.     As  previously   washmg 
stated,  never  completely  fill  the  filter  paper  and  wait  till  it 
has  entirely  drained  before  adding  more  liquid. 

Cover  the  funnel  with  an  ordinary  11  cm.  filter  paper  wet  with 
distilled  water. 

Place  the  funnel  hi  an  oven  to  dry  the  precipi-  Drying  of  the 

tate.  precipitate 

This  precipitate  cannot  be  ignited  with  the  paper  as  the  carbon 
from  the  filter  paper  will  reduce  copper  oxid  to  metallic 
copper  (see  reaction  III). 


EXPLANATORY  FACT 

22.  Cupric  hydroxid  must  be  completely  changed  by  boiling  to 
cupric  oxid. 

*  See  "Chemistry  of  the  Metals,"  Experiment  No.  232. 


C  — 

GRAVIMETRIC  ANALYSIS  49 

Place  a  perfectly  clean  three-inch  watch  glass  upon  a  circular 
piece  of  glazed  paper  about  six  inches  in  diameter. 

Separate  the  copper  oxid  from  the  filter  paper  in  the  following 
manner:  Remove  the  filter  paper  from  the  funnel.  Gently 
loosen  the  precipitate  adhering  to  the  sides  of  the  paper  by 
pressing  the  funnel-shaped  paper  with  Separatioa  of  ^ 
thumb  and  fingers.  With  great  care  empty  copper  ond  from  the 
the  bulk  of  the  contents  of  the  filter  paper  mter  **" 
upon  the  watch  glass.  Invert  the  filter  paper  over  the 
watch  glass  and  gently  rub  the  sides  together.  Avoid 
rubbing  hard  enough  to  detach  threads  of  the  paper. 
Cover  the  precipitate  with  an  inverted  six-inch  watch 
glass. 

Wipe  off  any  of  the  precipitate  that  has  adhered  to  the  glass 
funnel  and  drop  the  bit  of  paper  into  the  filter-paper 
cone. 

Fold  the  filter  paper,  which  should  be  flattened  into  the  shape 
of  a  quarter  circle,  in  halves  lengthwise,  fold  once  more  in 
the  same  way.  Roll  the  top  edge  and  secure  it  by 
fastening  around  it  loosely  a  platinum  wire  which  will 
then  come  into  contact  with  clean  paper 

Preparing  and  burning 
Only.  the    filter    paper    after 


Hold  the  folded  filter  paper  by  the  wire  over 

the  weighed  crucible,  which  should  stand 

on  a  piece  of  glazed  paper.     Burn  and  let  the  ash  fall  into 

the  crucible  (see  reaction  III). 
Moisten  the  residue  with  one  or  two  drops  of  nitric  add  to  dis- 

solve it  and    form   cupric    nitrate  *    (see  Treatment  of  any 

reaction  IV).  reduced  oxid 

To  avoid  loss  by  spattering,  heat  with  great  care  on  a  hot  plate 

to  drive  off  the  excess  of  nitric  acid  till  the  mass  is  dry  (see 

under  "Evaporation  of  Liquids,"  page  24). 
Place  the  crucible  on  its  side  on  a  clean,  clay  triangle  on  a 

ring  stand  and  heat  first  gently  and  then  to  redness  till  the 

cupric  nitrate  has  changed  into  black  cupric  oxidf  (see 

reaction  V). 

*  See  "Chemistry  of  the  Metals,"  Experiment  No.  91. 
t  See  "Chemistry  of  the  Metals,"  Experiment  No.  96. 


50  QUANTITATIVE  ANALYSIS 

Remove  the  lamp  and  when  the  crucible  is  nearly  cool,  place 
it  on  a  piece  of  glazed  paper  and  transfer  the  main  part  of 
the  precipitate  which  is  on  the  watch  glass  into  the  crucible. 
Brush  off  the  particles  remaining  on  the   Treatment  of  ^^ 
glass    with    a    small    brush    or    trimmed   entire  combined 
feather.     Finally  brush  into  the  crucible   precipltate 
any  particles  that  may  be  on  the  glazed  paper. 
Heat  strongly  for  a  few  more  minutes.  ignition 

Cool  in  the  desiccator.  cooling,  and 

Weigh  and  record  the  weight.  weighing 

Repeat  the  heating,  cooling  and  weighing  till 

the  weights  agree  within  three-tenths  of  a  milligram,  thus 
bringing  the  substance  to  a  "constant  weight." 


GRAVIMETRIC  ANALYSIS  51 


SAMPLE  CALCULATION 

The  per  cent,  of  Cu  in  CuS04.5  H2O. 

Weighing  tube  +  copper  sulfate  =  14.8345  grams 
Weighing  tube  —  charge  =  13.6405  grams 

Weight  of  charge  =    1.1940  grams 

Weight  of  crucible  with  cover  +  ash  -f  CuO  =  25.4987  grams 
Weight  of  crucible  with  cover  =  25.1190  grams 

Weight  of  ash  and  CuO  =    0.3797  grams 

Weight  of  ash  =    0.0002  grams 

Weight  of  CuO  =    0.3795  grams 

Atomic     weight  of  Cu     =  63.60 

"      "   O      =16.00 

Molecular      "      "  CuO  =  79.60 

The  ratio  of  Cu  to  CuO  ='  =  0.79897  +  or  0.7990. 


0.7990  is  the  "constant  factor"  that  represents  the  amount  of 
Cu  in  CuO. 

(Weight  of  CuO),  0.3795  grams,  multiplied  by  0.7990  (the  con- 
stant factor)  =  0.30322  gram  of  Cu  in  1.1940  grams  of  the  salt. 

0.30322^  100 


52  QUANTITATIVE  ANALYSIS 


CALCULATION  BY  LOGARITHMS 

The  logarithm  of  the  weight  of  CuO  minus  the  logarithm  of  the 
weight  of  the  original  charge  plus  the  logarithm  of  the  constant 
factor  of  the  amount  of  Cu  in  CuO  equals  the  logarithm  of  the  per 
cent,  of  Cu. 

Log.  of  (weight  of  CuO)  0.3795     =   9.57921-10 
Log.  of  (original  charge)  1.1940     =   0.07700 


9.50221-10 
Log.  of  (constant  factor)  0.79900  =   9.90255-10 

19.40476-20  or 
1.40476 

Anti-log  of  1.40476  =  0.2539  or  25.39%. 


GRAVIMETRIC  ANALYSIS  53 


DETERMINATION   OF  IRON 

IN  AN 

UNKNOWN,    SOLUBLE,    FERROUS    SALT    SIMILAR    TO 
FERROUS  SULFATE 

Iron  is: 

(a)  oxidized  in  the  presence  of  hydrochloric  acid  to  the  ferric 

state  by  nitric  acid,  HNO3; 
(6)  precipitated  by  ammonium  hydroxid,  NH4OH,  as  ferric 

hydroxid,  Fe(OH)3; 

(c)  converted  by  ignition  to,  and  weighed  as,  ferric  oxid,  Fe2O3. 

(Type  of  precipitate  which  by  ignition  loses  component 
water) ; 

(d)  calculated  as  per  cent,  of  iron. 

REACTIONS 

I.  6FeS04  +  8HNO3  =  2Fe2(SO4)3  +  2Fe(NO3)3  +  2  NO 

+  4H2O. 
II.  2  FeS04  +  NO  =  (FeS04)2NO. 

III.  Fe2(S04)3  +  6  NH4OH  =  2  Fe(OH)3  +  3  (NH4)2SO4. 

IV.  Fe(N03)3  +  3NH4OH  =  Fe(OH)3  +  SKH^Os. 
V.  2  Fe(OH)3  +  A  =  F^O3  +  3  H2O. 

ADDITIONAL  REACTIONS  POSSIBLE  IN  THE  PROCESS 

VI.  2  FeSO4  +  O  =  Fe2O(SO4)2. 

VII.  3  Fe20(S04)2  +  6  HC1  =  2  Fe2(SO4)3  +  2  FeCla  +  3  H20. 
VIII.  3  FezOs  +  C  =  CO  +  2  Fe3O4. 
IX.  2Fe304  +  O  =  3Fe203. 

X.  Fe(OH)3  +  3  NH4C1  +  A  =  FeCl3  (partly  volatile) 

+  3NH3  +  3H2O. 


54  QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  IRON 

PROCEDURE 

Weigh  into  two  350  c.c.  beakers  (No.  3),  two  weighing  the 

portions  of  the  salt  of  1.5  grams  each.  charge 

Dissolve  in  100  c.c.  of  distilled  water  to  which   Dissolving  the 

4  c.c.  of  concentrated  hydrochloric*  add23  substance 

have  been  added. 
Cover  the  beakers  with  watch  glasses,  bring  the   oxidizing  the  iron 

solution  to  a  boil  and  add  to  each  2  c.c.  of  to  *"»  ferric  state 

nitric  f  aa'd.24*25 
Again  cover  the  beakers  and  keep  them  at  the  boiling  point  for 

about  fifteen  minutes.    The  solutions  will  now  show  the 

presence  of  ferric  salts  by  the  clear  yellow  or  red  color 


EXPLANATORY  FACTS 

23.  Unless  there  is  free  acid  present,  ferrous  iron  will  not 
wholly  oxidize  and  basic  ferric  salts  will  precipitate  (see  reactions 
VI  and  VII). 

24.  The  iron  must  be  oxidized  (see  reaction  I)  before  precipi- 
tation as  the  ammonia  does  not  completely  precipitate  ferrous 
salts  and,  even  if  it  did,  such  a  precipitate,  t  owing  to  its  tendency 
to  oxidize,  would  be  of  indefinite  composition. 

25.  The  brown  coloration  that  appears  when  nitric   acid  is 
added  is  due  to  the  combination  of  NO  with  the  ferrous  sulfate 
in  the  solution  (see  reactions  I  and  II).     Upon  heating,  this  com- 
pound is  destroyed  and  the  nitric  oxid  escapes. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  157. 
t  "Chemistry  of  the  Metals,"  Experiment  No.  155. 
t  "Chemistry  of  the  Metals,"  Experiments  Nos.  158  and  160. 


GRAVIMETRIC  ANALYSIS  55 

Test  for  complete  oxidation  by  adding  a  drop  or  two  more  of 
nitric  add  which,  if  no  ferrous  iron  remains,  will  produce 
no  further  brown  coloration  (see  reaction  II). 

Allow  the  solution  to  cool. 

While   stirring,    add   a   slight   excess   of   am-  Precipitation  of 
monia 26*27  (test  by  the  odor).  ferric  tydroxid 

Heat  to  incipient  boiling  with  constant  stirring.28 

Allow  the  precipitate*  to  settle. 

With  hot  water,  wash  several  times  by  decantation  through  the 
filter  into  a  500  c.c.  beaker. 

Transfer  all  of  the  precipitate  to  the  filter  paper  Filtration  of  the 
and  wash  with  hot  water29*30  until  a  few  hydrorid 
drops  of  the  filtrate  no  longer  give  a  test  washing  the 
for  HC1  with  AgN03  solution.!  hydroxid 


EXPLANATORY  FACTS 

26.  Unless  all  the  iron  is  precipitated  as  quickly  as  possible 
throughout  the  solution,  SO4  is  lost,  as  basic  ferrous  sulfate,  by 
being  "dragged  down"  in  neutral  zones. 

27.  The  first  action  upon  the  addition  of  ammonia  is  the  neu- 
tralization of  the  free  acid  in  the  solution,  which  results  in  the 
formation  of  ammonium  nitrate  and  ammonium  chlorid. 

28.  In  distinction  from  chromium  and  aluminium  hydroxid, 
ferric  hydroxid  is  practically  insoluble  in  an  excess  of  ammonia. 

29.  Ferric  hydroxid  may  be  changed  to  ferric  chlorid  if  heated 
with  any  ammonium  chlorid  that  has  been  left  by  insufficient 
washing.     As  ferric  chlorid  is  somewhat  volatile  this  would  re- 
sult in  a  loss  of  iron  (see  reaction  X). 

30.  In  order  to  avoid  the  solvent  action  of  ammonia  upon  the 
glass,  filtration  must  be  begun  at  once  after  the  precipitate  has 
settled.     Once  begun,  the  filtration  and  washing  must  be  com- 
pleted without  delay. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  167. 
f  "Chemistry  of  the  Metals,"  Experiment  No.  49. 


56  QUANTITATIVE  ANALYSIS 

Dry   the   precipitate  in  the  oven  and  ignite  to   a   constant 
weight.31*32    There  should  not  be  a  greater 

7  \.         .,  .,  ,.  .„.  Drying,  igniting  and 

variation  than  three-tenths  of  a  milligram,   weighing  the  pre- 
Make  the  final  ignition  in  a  covered  crucible  ciPitate 

over  a  blast  lamp. 
Report  the  per  cent,  of  iron. 


EXPLANATORY  FACTS 

31.  Proper  ignition  of  the  precipitate  (see  reaction  V)  changes 
it  to  Fe2Os.    Under  reducing  conditions,  however,  reaction  VIII 
may  take  place  and  form  FesCX  or  even  metallic  iron.     If  sufficient 
air  is  admitted  to  the  crucible  during  ignition,  both  of  these  prod- 
ucts will  change  to  Fe2Os  (see  reaction  IX). 

Reaction  V  is  not  completed  without  the  use  of  the  blast 
lamp. 

32.  Fe^Os  is  hygroscopic:   to  avoid  the  absorption  of  water,  it 
should  be  weighed  as  quickly  as  possible. 


GRAVIMETRIC  ANALYSIS  57 


DETERMINATION  OF  THE  ACID  RADICAL  SO4 

IN  AN 

UNKNOWN,    SOLUBLE,    FERROUS    SALT    SIMILAR    TO 
FERROUS   SULFATE 

SO4  is: 

(a)  precipitated   in   the   presence   of    hydrochloric   acid   by 
barium  chlorid,  BaCk,  as  barium  sulfate,  BaSO4; 

(6)  weighed  as  barium  sulfate,  BaSO4; 
(c)  calculated  as  per  cent,  of  S04.  , 


REACTION 
I.   (NH4)2S04  +  BaCl2  =  BaS04  +  2  NH4C1. 

ADDITIONAL  REACTIONS  POSSIBLE  IN  THE  PROCESS 

II.  BaSO4  +  2  C  =  BaS  +  2  C02. 

III.  BaS  +  2O2  =  BaS04. 

IV.  BaS  +  H2S04  =  BaS04  +  H2S. 


58  QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  THE  ACID  RADICAL  SO4 


PROCEDURE 


Add  hydrochloric  acid  in  slight  excess  to  the  filtrate  from  ferric 
hydroxid.33 

On  a  water  bath,  evaporate 34  this  solution  to  dryness  in  a  six- 
inch  evaporating36  dish  (see  "Evaporation  of  Liquids,"  page 
24).     Cover  with  a  watch  glass  which  should  rest,  not  upon 
the  rim  of  the  dish,  but  upon  a  glass  triangle  made  of  bent 
stirring  rod.     This  allows  steam  to  escape,   Evaporation  of 
catches  the  spatters  and  prevents  dust  from   the  filtrate 
falling  in. 


EXPLANATORY  FACTS 

33.  It  was  necessary  to  precipitate  the  iron  before  the  de- 
termination of  sulfur,  as  (1)  the  iron  tends  to  contaminate  the 
sulfate    and    (2)    barium    sulfate    is    slightly   soluble   in   ferric 
chlorid. 

34.  During  evaporation,  both  the  nitric  and  the  hydrochloric 
acids  are  decomposed  and  volatilized.     Should  any  nitric  acid  re- 
main the  barium  salt  of  this  acid  would  form  and  contaminate  the 
precipitate  and  the  final  per  cent,  would  be  too  high.    Nitrates  of 
the  alkalies  and  iron  are  also  particularly  liable  to  be  dragged 
down  with  barium  sulfate. 

35.  Evaporation  is  hastened  by  increasing  the  top  surface  of  the 
heated  liquid.    Broad,  shallow  dishes,  then,  are  more  suited  to 
evaporations  than  those  that  are  high  and  narrow. 

Rapid  evaporation  may  be  obtained  by  blowing  a  gentle  cur- 
rent of  warm,  dry  air  onto  the  surface  of  the  liquid. 


GRAVIMETRIC  ANALYSIS  59 

Dissolve  the  residue  in  100  c.c.  of  water  and,  if  necessary,  filter 
the  solution.  This  evaporation  is  to  remove  the  nitric 
acid  which  is  here  in  the  form  of  ammonium  nitrate36 
and  which,  in  the  previous  process,  was  used  to  oxidize 
the  iron  (see  reaction  I  under  "  The  Determination  of 
Iron"). 

Transfer  quantitatively  to  a  beaker  and  add  5  c.c.  of  dilute 
hydrochloric 37  acid. 

Heat   the   covered   solution  to   boiling38   and,   precipitation  of  the 
while  stirring,  add  slowly,39  drop  by  drop,   barium  suifate 


EXPLANATORY  FACTS 

36.  Barium  suifate  possesses  the  quality  of  dragging  down  with 
it  compounds,  even  those  that  are  ordinarily  soluble.     These  are 
not  easily  removed  by  washing  and,  therefore,  lead  to  inaccurate 
results. 

37.  (a)  As  hydrochloric  acid,  even  when  it  is  dilute,  dissolves 
some  barium  suifate,  only  the  smallest  excess  is  added.    The 
presence  of  an  excess  of  BaCk  lessens  this  solubility;   a  large 
excess,  however,  should  be  avoided  as  there  is  danger,  especially 
if  the  reagent  is  added  quickly,  of  some  of  it  being  precipitated 
with  the  barium  suifate. 

(6)  When  sulfuric  acid  is  precipitated  by  barium  chlorid,  some 
of  the  chlorid  is  often  "dragged  down"  by  the  barium  suifate. 
As  barium  suifate  is  somewhat  soluble,  these  two  sources  of  error 
under  favorable  conditions  almost  counteract  each  other. 

38.  Barium  suifate  formed  in  hot  acid  solution  is  in  the  best 
condition  to  be  retained  by  the  filter  paper.     If  the  solution  and 
the  barium  chlorid  were  cold,  there  would  be  formed  a  finely 
divided  precipitate  which  would  have  to  stand  hours  before  it 
could  be  filtered. 

39.  If  the  reagent  is  added  quickly,  barium  chlorid  as  well  as 
barium  suifate  will  be  precipitated. 


60  QUANTITATIVE  ANALYSIS 

not  exceeding  5  c.c.  per  minute,  a  moderate  excess  of  boiling 
barium  chlorid*  solution.40 

After  the  precipitate  has  settled,  add  a  few  more  drops  of  barium 
chlorid.  Keep  the  liquid  nearly  at  the  boiling  point  and 
stir  occasionally  during  one-half  hour  or  more. 

Test  for  complete  precipitation  by  again  adding  a  few  drops  of 
the  barium  chlorid. 

Pour  the  supernatant  liquid  through  a  special   Filtration  of  the 
filter,41  disturbing  the  precipitate  as  little   sulfate 
as  possible. 

Wash  the  precipitate  twice  by  decantation  with  hot  water  acidi- 
fied with  hydrochloric  acid.     Then  wash   washing  the 
with  hot  water  alone  and  transfer  the  pre-  sulfate 
cipitate  to  the  filter. 

Wash  till  the  filtrate  gives  with  silver  nitrate  Testing  the  gitrate 
no   reaction   for   chlorids.42     The   filtrate  for  complete 
must  be  clear.  washing 


EXPLANATORY  FACTS 

40.  A  soluble  barium  salt,  like  barium  chlorid,  or  a  soluble 
sulfate,  makes  the  barium  sulfate  less  soluble.     For  this  reason, 
it  is  desirable  to  have  an  excess  of  barium  chlorid,  but  as  barium 
chlorid  is  carried  down  with  barium  sulfate,  a  large  excess  should 
be  avoided. 

41.  Some  of  the  barium  sulfate  may  run  through  an  ordinary 
filter. 

42.  The  absence  of  chlorids  proves  that  the  washing  is  com- 
plete. 


*  "Chemistry  of  the  Metals,"  Experiment  No.  232. 


GRAVIMETRIC  ANALYSIS  61 

Dry  in  the  oven,43  ignite44  and  weigh.     Before  the  second  igni- 
tion add  one  drop  of  concentrated  sulfuric  Drying,  ignition 
acid.    Be  sure  that  a  constant  weight  is  a°d  weighing 
reached. 

Make  the  final  ignition,  with  the  cover  on,  over  the  blast  lamp. 


EXPLANATORY  FACTS 

43.  With  proper  precautions  this  precipitate  can  be  ignited 
without  putting  it  into  the  drying  oven  (see  under  "Ignition  to  a 
Stable  Compound, "  page  25,  also  pages  39  and  40). 

44.  Ignite  slowly  to  properly  burn  the  filter  paper.    Barium 
sulfate  at  a  high  temperature  may  be  reduced  by  the  carbon  of 
the  filter  paper  to  barium  sulfid.     Although  sufficient  air  will 
usually  change  the  sulfid  back  to  sulfate,  the  change  is  sure  to 
take  place  if  sulfuric  acid  is  added. 


62  QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  CHLORIN* 

IN 
AN  UNKNOWN,   SOLUBLE  CHLORID 

(Corresponding  to  the  general  formula  MCI  or  MC12) 


Chlorin  is: 

(a)  precipitated  by  silver  nitrate,  AgNOs,  as  silver  chlorid, 

AgCl,f  a  type  of  precipitate  which  must  be  removed 

from  the  filter  paper  before  ignition; 
(6)  weighed  as  silver  chlorid; 
(c)  calculated  as  per  cent,  chlorin. 


REACTIONS 

I.  MCI  +  AgN03  =  AgCl  +  MN03  or 
II. :  MC12  +  2  AgN03  =  2  AgCl  +  M(N03)2. 


*  Reversed,  except  in  certain  special  cases,  this  process  may  be  used  for  the 
determination  of  silver  in  alloys,  etc. 

t  Unless  it  fuses,  AgCl  is  unchanged  by  ignition,  but  if  it  is  in  contact  with  the 
filter-paper  when  it  is  ignited,  it  is  reduced  by  the  carbon  of  the  filter  paper  to 
netallic  silver.  Therefore  the  bulk  of  the  AgCl  is  removed  from  the  paper,  which 
is  burned  separately. 


GRAVIMETRIC  ANALYSIS  63 


DETERMINATION   OF   CHLORIN 

All  silver  precipitates  and  filtrates  containing  silver  salts  are 
to  be  put  into  a  side-shelf  bottle  marked  "  Silver  Residues." 


PROCEDURE45 

Weigh  into  two  350  c.c.  beakers  two  portions  of 

the  chlorid,  0.3  of  a  gram  each.  charge 

Dissolve  in  100  c.c.  of  cold  distilled  water.46  Dissolving  the 

Add  3  c.c.  of  dilute  nitric47  add  (sp.  gr.  1.2).  substance 


EXPLANATORY  FACTS 

45.  In  this  determination  avoid  exposure  to  bright  light..    It 
changes  the  silver  chlorid  from  white  to  violet,  which  shows  a  de- 
composition to  a  lower  chlorid  with  a  loss  of  chlorin. 

Although  later,  when  the  hydrochloric  and  nitric  acids  are 
added,  any  chlorin  that  has  been  lost  is  replaced  (see  explanatory 
fact  52),  it  is  as  well  to  avoid  the  necessity  of  correcting  previous 
error. 

46.  Silver  chlorid  is  almost  insoluble  in  cold  water,  but  more 
soluble  in  hot  water. 

47.  Nitric  acid  (1)  decreases  the  solubility  of  silver  chlorid, 
(2)  keeps  in  solution  any  other  compounds  that  might  be  carried 
down  with  the  silver  chlorid  and  (3)  overcomes  the  tendency 
which  silver  chlorid  has  in  cold  water  of  changing  to  a  colloidal 
state,  a  condition  in  which  the  precipitate  will  pass  through  the 
filter. 

The  amount  of  nitric  acid  is  limited  to  this  amount  as  silver 
chlorid  is  somewhat  soluble  in  concentrated  nitric  acid. 


64  QUANTITATIVE  ANALYSIS 

Add  silver  nitrate  solution,  drop  by  drop,  with  constant  stirring, 
until   there   is  no   further    precipitation.*   Precipitation  of 
There  should  be  a  slight  excess  of  the  silver  ** chlorid 
nitrate.48 

Heat  nearly  to  the  boiling  point 49  and  stir  constantly  until  the 
precipitate  has  coagulated  and  the  liquid  is  clear. 

Place  the  solution  in  a  dark  place  to  settle. 

Use  filter  papers  9  cm.  in  diameter. 

Pour    the   liquid   through    the   filter   without   Filtration  of  the 
greatly  disturbing  the  precipitate.  silver  chlorid 

Catch  the  filtrate  in  a  500  c.c.  beaker. 

Wash  the  precipitate  in  the  beaker  three  times  with  cold  water 
slightly  acidified  with  nitric  acid50,  decant-   washing  the 
ing  the  liquid  through  the  filter.  chlorid 


EXPLANATORY  FACTS 

48.  An  excess  of  silver  nitrate,  as  in  the  case  of  nitric  acid,  de- 
creases the  solubility  of  silver  chlorid  and  aids  in  the  coagulation 
of  the  precipitate. 

If  the  precipitate  is  not  flocculent  there  is  probably  not  a  suffi- 
cient excess  of  silver  nitrate. 

49.  Do  not  heat  to  boiling  before  the  silver  nitrate  is  added  in 
excess,  as  some  of  the  unprecipitated  chlorin  might  be  liberated 
from  the  original  salt  (KC1,  NaCl,  etc.)  by  the  action  of  the  hot 
nitric  acid. 

The    heat    is    necessary    to    make    all    the   precipitate    floc- 
culent. 

50.  Nitric  acid  prevents  the  coagulated  precipitate  from  return- 
ing to  a  colloidal  state. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  49. 


GRAVIMETRIC  ANALYSIS  65 

Transfer  the  precipitate  to  the  filter  and  continue  the  washing 
with  cold  water  acidified  with  nitric  acid  Testing  ^^  ffltrate 
until  a  drop  of  hydrochloric  acid  in  3  c.c.  of   for  complete  wash- 
the  wash  water  shows  no  turbidity.51     Be   "* 
sure    to   wash    the    filter    paper    clean    above    the    pre- 
cipitate. 

Wash  twice  with  cold  water  or  a  mixture  of  alcohol  and  water, 
half  and  half,  to  remove  the  nitric  acid.  Test  for  the 
absence  of  nitric  acid  with  litmus  paper.  Save  the  filtrate 
for  silver  recovery. 

Dry  the  precipitate  in  the  oven  at  100°  C.     This  precipitate 
cannot  be  ignited  with  the  filter  paper,  as   Drying  the  pre- 
the  carbon  from  the  paper  will  reduce  silver  «Pitate 
chlorid  to  metallic  silver. 

Place  a  perfectly  clean  three-inch  watch  glass  upon  a  circular 
piece  of  glazed  paper  about  six  inches  in  diameter. 

Separate  the  silver  chlorid  from  the  filter  paper  in  the  following 
manner:  Remove  the  filter  from  the  funnel.  Gently 
loosen  the  precipitate  adhering  to  the  sides  Special  precautions 
of  the  paper  by  pressing  the  funnel-shaped 
paper  with  the  thumb  and  fingers.  With 
great  care,  empty  the  contents  of  the  filter  paper  upon  the 
watch  glass.  Invert  the  filter  paper  over  the  watch  glass 
and  carefully  rub  the  sides  together.  Avoid  rubbing  hard 
enough  to  detach  threads  of  the  paper.  Cover  the  pre- 
cipitate with  an  inverted  six-inch  watch  glass. 


EXPLANATORY  FACT 


51.  Mixed  with  the  AgCI  are  nitrates  of  the  alkali  metals  that 
were  in  combination  in  the  original  salt  and  nitrate  of  silver. 
The  absence  of  the  latter  in  the  HC1  test  indicates  that  the  other 
soluble  nitrates  are  also  washed  away. 


66  QUANTITATIVE  ANALYSIS 

After  removing  the  precipitate,  press  the  filter  paper  into  the 
shape  of  a  quarter  circle,  fold  in  halves  lengthwise,  fold  once 
again  in  the  same  way.  Roll  the  top  edge  and  secure  it  by 
fastening  around  it  loosely  a  platinum  wire.  If  there  is  any 
silver  chlorid  on  the  edge  of  the  paper  the  heat  will  reduce 
it  to  metallic  silver,  which  will  alloy  with  the  platinum.  This 
will  break  the  wire  and  spoil  the  determination  as  well. 

Hold  the  folded  filter  paper  by  the  wire  over  a  weighed  porcelain 
crucible,  which  should  be  on  a  piece  of  glazed  paper.  Burn 
and  let  the  ash  fall  into  the  crucible. 

Place  the  crucible  on  its  side  on  a  clay  triangle.     As  silver 
chlorid  volatilizes  at  rather  a  low  temperature,  apply  the 
Bunsen  flame  only  on  that  part  of  the  cru-  ignition  of  the 
cible  upon  which  there  rest  particles  of  chlorid 
carbonaceous  matter.     When  these  have  whitened,  remove 
the  lamp  and  cool  the  crucible. 

Add  to  the  ash  two  drops  of  concentrated  nitric  acid  to  dissolve 
the  metallic  silver*  and  then  two  drops  Treatment  of  any 
of  concentrated  hydrochloric  acid  to  convert  reduced  silver 
it  to  the  chlorid. 

Evaporate  to  dry  ness  on  the  water  bath. 

Transfer  the  main  part  of  the  precipitate  which  is  on  the  watch 
glass  into  the  crucible.  Hold  the  watch  glass  over  the 
glazed  paper  and  brush  off  into  the  crucible  the  remaining 
particles  with  a  small  brush  or  trimmed  feather.  Finally 
brush  into  the  crucible  any  particles  that  may  be  on  the 
glazed  paper. 

Add  to  the  precipitate  two  drops  of  concentrated  nitric  acid  and 
two  drops  of  concentrated  hydrochloric  52  acid. 


EXPLANATORY  FACT 

52.  It  is  at  this  point  that  the  silver  chlorid  which  has  been 
reduced  to  metallic  silver  by  sunlight  is  dissolved  and  changed 
back  to  the  chlorid 

*  "Chemistry  of  the  Metals,"  Experiment  No.  46. 


GRAVIMETRIC  ANALYSIS  67 

Evaporate  to  dryness  on  the  water  bath. 

Heat  with  great  care  until  the  silver  chlorid  barely  begins  to  fuse 
at  its  edges.53  If  it  is  heated  too  strongly  the  deter- 
mination will  be  ruined.  weighing  the 

Cool  and  weigh.  cMorid 

Heat,  cool  and  weigh  until  a  constant  weight  is  obtained. 


EXPLANATORY  FACT 


53.  Silver  chlorid  slightly  decomposes  and  volatilizes  at  its 
temperature  of  fusion,  460°  C. 


QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  CALCIUM  AND  MAGNESIUM 

IN 
MIXED  CARBONATES54 

1.  The  soluble  constituents   are  dissolved  from  the  mineral, 
the  solution  is  evaporated  to  dryness  and  the  residue  dehydrated. 
This  leaves  insoluble  siliceous  matter. 

2.  Iron  and  aluminium  are  precipitated  by  ammonium  hydroxid, 
NH4OH,  as  ferric  hydroxid,  Fe(OH)3,  and  aluminium  hydroxid, 
A1(OH)3. 

3.  Manganese,  if  present,  is  precipitated  by  ammonium  hy- 
droxid, after  oxidation  with  bromin,  as  an  oxid  of  varying  com- 
position. 

1,  2  and  3  are  not  weighed.     They  are  simply  to  be  removed 
and  rejected  before  the  calcium  and  magnesium  are  precipitated. 

4.  Calcium  is: 

(a)  precipitated  by  freshly  prepared  ammonium  oxalate  solu- 
tion (NHOzCaO^  as  calcium  oxalate,  CaC2O4; 
(6)  changed  by  ignition  to  calcium  oxid  and  weighed  as  such; 
(c)  calculated  as  per  cent,  calcium  oxid. 

5.  Magnesium  is: 

(a)  precipitated  by  microcosmic  salt,  NaNEUHPO^  as  crys- 
talline magnesium  ammonium  phosphate,  MgNH4P04; 

(6)  changed  by  ignition  to  magnesium  pyrophosphate,  MgzPzOy, 
and  weighed  as  such; 

(c)  calculated  as  per  cent,  of  magnesium  oxid. 


EXPLANATORY  FACT 


54.  Such  a  mixture  is  found  in  dolomite  (magnesium  lime- 
stone). This  is  one  of  the  common  minerals  whose  composition 
is  CaCOs  .  MgCOs  in  varying  proportions.  It  may  also  contain 
manganese,  aluminium,  iron,  silica,  etc. 


GRAVIMETRIC  ANALYSIS 


REACTIONS 
Calcium  and  Magnesium 

I.  CaCO3  +  2  HC1  =  CaCl2  +  H2O  +  CO2. 

II.  MgCO3  +  2  HC1  =  MgCl2  +  H2O  +  CO2. 

III.  H4Si04  +  A  =  2  H20  +  Si02. 

IV.  CaCl2  +  (NH4)2C204  =  CaC2O4  +  2  NI^Cl. 
V.  CaC204  +  2  HC1  =  CaCl2  +  H2C2O4. 

VI.  CaCl2  +  H2C204  +  2  NH4OH  =  CaC2O4  +  2  NH4C1 

+  2H20. 

VII.  CaC204  +  A  =  CaO  +  CO2  +  CO. 
VIII.  MgCl2  +  NaNH4HP04  =  MgHP04  +  NaCl  +  NH4CL 
IX.  MgHP04  +  NH3  =  MgNH4P04. 
X.  2  MgNH4P04  +  A  =  Mg2P207  +  2  NH3  +  H20. 


70  QUANTITATIVE  ANALYSIS 


DOLOMITE 


PROCEDURE 


Weigh  into  350  c.c.  beakers  two  portions  of  the   weighing  the 
finely  ground  mineral  of  1.5  grams  each.   char«e 


REMOVAL  OF  THE  SILICEOUS  MATTER55 

Treat  in  covered  beakers  with  hydrochloric  acid*   Dissolving  the 
(1.12  sp.  gr.)  until  action  ceases  (see  re-   substance 
actions  I  and  II).    . 

Transfer  quantitatively  into  evaporating  dishes  and  evaporate 
to  dryness  on  a  water  bath.  Dehydration  of  the 

Heat  for  two  hours  at  130°  C.  (see  reaction  III).    sUicic  acid 

Cool  and  add  to  the  residue  a  few  drops  of  concentrated  hydro- 
chloric acid. 


EXPLANATORY  FACT 

55.  The  siliceous  residue,  composed  of  quartz,  clay,  etc.,  is 
largely  insoluble  in  the  dilute  acid.  Some  soluble  silicic  acid 
may,  however,  pass  into  solution.  It  must  be  heated  at  130°  C. 
to  change  it  to  insoluble  silica,  SiC>2,  which  can  be  filtered  off. 

The  mineral  could  be  dissolved  directly  in  an  evaporating  dish 
but  it  is  somewhat  easier  to  judge  of  cessation  of  solution  if  the 
action  takes  place  in  glass. 

*  "  Chemistry  of  the  Metals,"  Experiment  No.  240. 


GRAVIMETRIC  ANALYSIS  71 

Warm  cautiously  and  then  add  20  c.c.  of  hydrochloric  acid  (1.12 

sp.  gr.)  and  20  c.c.  of  water. 
While  warm,  filter  into  a  beaker. 
Wash  the  residue  on  the  filter  paper  with  hot  Removal  of  the 

water  till  it  is  free  from  hydrochloric  acid,    s^ceous  residue 

when  it  may  be  rejected. 


REMOVAL  OF  ALUMINIUM  M  AND  IRON  w  (AND  MANGANESE) 

If  a  qualitative  test  has  shown  the  presence  of  manganese,58  it  is 
precipitated  at  this  point  by  adding  bromin  water  to  the 
filtrate  from  the  silica  till  a  permanent  yel-  Remo7al  of  ^^ 
low  color  remains.     Then  proceed  with  the  aluminium  and 
addition  of  NH4C1  and  NH4OH  as  in  the  man*anese 
following  paragraph. 

If  manganese  is  absent  add  a  few  drops  of  nitric  acid  to  the  fil- 
trate from  the  silica  and  boil  to  be  sure  that  the  iron  is  all 
oxidized.  Add  the  least  possible  excess  of  ammonium 
hydroxid.  Gently  boil  the  solution  till  it  is  but  faintly 
ammoniacal. 

Quickly59  filter  the  precipitated  iron  and  aluminium  hydroxid 
(and  perhaps  manganese)  and  wash  three  or  four  times 
with  hot  water. 

Mark  the  filtrate  and  washings  "A." 


EXPLANATORY  FACTS 

56  and  57.   See  notes  under  aluminium  and  iron  determina- 
tions. 

58.  It  is  necessary  to  add  bromin  to  oxidize  the  manganese, 
which,  with  ammonia,  forms  a  hydrated  dioxid.     This  hydrated 
dioxid  of  manganese  is  thus  removed  with  the  iron  and  alu- 
mina. 

59.  Filter  quickly  to  avoid  action  of  ammonia  upon  the  glass 
and  also  to  avoid  the  absorption  of  carbon  dioxid. 


72  QUANTITATIVE  ANALYSIS 

Dissolve  *°  the  precipitate  of  iron,  aluminium   solution  of  the  iron, 

(and  manganese)  on  the  filter  paper  with 

hot,  dilute  hydrochloric  add,  receiving  this    tates 

solution  in  a  separate  beaker. 

Thoroughly  wash  the  acid  solution  out  of  the  paper. 
Test  washings  with  litmus  paper. 
If  manganese  is  not  present,  add  a  slight  excess  Re  eci  itatLon  of 

of   ammonia    water    to    reprecipitate    the  the  iron,  aluminium 

iron  and  aluminium  from  this  acid  solu-  andmansanese 

tion. 
If  manganese  is  present,  again  add  a  few  drops  of  bromin 

water  before  adding  the  ammonia. 
Boil  till  only  faintly  ammoniacal. 

Filter  and  wash  free  from  chlorids.     Reject  the  precipitate. 
Mark  the  filtrate  and  washings  "B." 


EXPLANATORY  FACT 

60.  This  re-solution  and  reprecipitation  is  necessary  to  com- 
pletely separate  the  small  quantity  of  calcium  which  may  have 
precipitated  as  carbonate,  due  to  absorption  of  atmospheric  C02 
by  the  alkaline  solution. 


GRAVIMETRIC  ANALYSIS  73 


DETERMINATION  OF  THE  CALCIUM 

This  element  is  now  contained  in  the  filtrate  and  washings  "A" 
from  the  original  iron  and  aluminium  precipitate  and  in  the  filtrate 
and  washings  "B"  from  the  second  precipitation  of  iron  and 
aluminium  (and  possibly  of  manganese). 

Combine  the  filtrates  "A"  and  "B"  and  heat  to  boiling. 

Evaporate  to  250  c.c.  if  it  exceeds  this  volume. 

Make  alkaline  with  ammonium  hydroxid. 

To  the  boiling  ammoniacal  liquid,  add  slowly,  with  stirring,  a 
moderate    excess    of   warm,    freshly   pre-  precipitation  of  the 
pared 61  ammonium  oxalate 62  solution  *  (see  "fc"™  <*aiate 
reaction  IV). 

Heat  to  boiling  for  a  few  minutes  and  let  the  precipitate  settle 
for  half  an  hour  or  more. 

Decant  the  liquid,  but  do  not  remove  the  precipitate  to  the 
filter,  as  it  is  to  be  redissolved. 

Wash  the  precipitate  by  decantation  three  or  four  times  with 
hot  water  till  free  from  ammonium  chlorid,   washing  by  de- 
testing the  wash  water  with  silver  nitrate 
acidified  with  nitric  acid. 


EXPLANATORY  FACTS 

61.  Ammonium  oxalate  decomposes  slowly  and  ammonium  car- 
bonate is  one  product  of  this  decomposition. 

62.  Calcium  oxalate  is  somewhat  dissolved  by  magnesium  chlorid 
solution;  therefore,  enough  ammonium  oxalate  should  be  added 
to  convert  to  oxalate  the  magnesium  as  well  as  the  calcium. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  242. 


74  QUANTITATIVE  ANALYSIS 

Test  the  filtrate  for  complete  precipitation  with  a  few  drops  of 
ammonium  oxalate  and  let  it  stand.     This  filtrate,  when 
combined  with  the  filtrate  obtained  from   Treatment  of  the 
the   reprecipitated    calcium    oxalate    (see  ^h^precf^tation 
below),  if  perfectly  clear,  is  ready  for  the   of  the  magnesium 
magnesium  determination. 

Without  delay,  slightly  acidify  the  filtrate,  to  prevent  the  solvent 
action  of  the  alkali  upon  the  glass. 

If  the  filtrate  is  more  than  150  c.c.  in  volume,  evaporate  it  on 
the  water  bath.  The  precipitate,  if  the  original  sample 
contained  much  magnesium,  will  contain  some  magnesium 
oxalate  with  the  calcium  oxalate. 

Purify63  the  calcium  oxalate  as  follows:  redissolve  the   pre- 
cipitate on  the  filter  and  in  the  beaker  by  pm^^^  of  ^ 
pouring   warm,    dilute    hydrochloric    acid  first  calcium  ox- 
(see  reaction  V)  four  or  five  times  through   alate  precipltate 
the  filter  into  the  beaker  containing  the  precipitate.     After 
the  calcium  oxalate  is  all  dissolved,  wash  the  filter  with 
ammonium  hydroxid.     Dilute  the  solution  to  about  250  c.c. 
Heat  and  make  it  slightly  alkaline  with  ammonium  hydroxid 
(see  reaction  VI).    Add  5  c.c.  of  the  ammonium  oxalate 
solution64  and  let  it  stand  for  at  least  half  an  hour. 


EXPLANATORY  FACTS 

63.  Although  magnesium  oxalate  is  relatively  soluble,  espe- 
cially in  the  presence  of  ammonia  salts,  yet  as  calcium  oxalate  may 
drag  down  some  magnesium  oxalate,  a  re-solution  and  reprecipi- 
tation  is  necessary.     On  second  precipitation,  there  is  relatively 
so  little  magnesium  present  that  none  of  it  is  contained  in  the 
calcium  oxalate  precipitate. 

64.  This  ammonium  oxalate  is  added  to  insure  the  presence  of 
an  excess  of  the  reagent,  the  best  condition  for  the  precipitation 
of  the  calcium. 


GRAVIMETRIC  ANALYSIS  75 

Filter  through  the  same  filter  paper  as  used  Filtration  ^  wash. 

before.  ing  of  the  reprecipi- 

Wash  the  precipitate  with  hot  water.  teted  calcium  oxalate 

Test  the  filtrate  for  chlorids  and,  when  the  precipitate  is  free, 
slightly  acidify  the  filtrate  and  add  it  to  the  first  filtrate 
containing  the  bulk  of  the  magnesium  oxalate.  The  com- 
bined filtrates  should  not  exceed  200  to  300  c.c. 

Dry  the  precipitated  calcium  oxalate  in  the  oven. 

Ignite  in  a  platinum 65  crucible.     Heat  strongly  Drying,  ignition 
for  about  ten  minutes.  and  weighing 

Finish  with  a  blast  lamp  for  ten  minutes66  (see  reaction  VII). 

Repeat  until  a  constant  weight  is  obtained. 


EXPLANATORY  FACTS 

65.  A  higher  temperature  than  that  which  is  needed  by  many 
precipitates  is  required  for  the  complete  conversion  of  the  calcium 
oxalate  to  calcium  oxid.     Although  a  porcelain  crucible  may  be 
used,  in  this  case  one  of  platinum  is  better. 

66.  Weigh  quickly,  as  calcium  oxid  absorbs  moisture  from  the 
air. 


76  QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  MAGNESIUM 
METHOD  OF  WOLCOTT  GIBBS 

Concentrate67  the  filtrates  containing  the  magnesium  (and 
ammonium)  salts  to  about  200  c.c.  and  bring  to  a  boil. 

Add  to  the  almost  boiling  solution  several  drops  of  methyl 
orange  and,  to  make  neutral,  add  ammonia,  drop  by  drop, 
till  the  solution  just  becomes  yellow.  Keep  hot. 

Add  a  normal  solution  of  microcosmic  salt,  ^60^^^,,  of  ti,e 
NaNH4PO4,*  till  no  further  precipitation  magnesium  ammo- 
takes  place68  (see  reaction  VIII).  *•*  phosphate 

While  stirring,  add  a  volume  of  ten  per  cent,  ammonia  equal  to 
one-third  that  of  the  hot  solution69  (see  reaction  EX). 


EXPLANATORY  FACTS 

67.  If  during  concentration  any  magnesium  oxalate  settles  out, 
decant  the  liquid  into  another  beaker,  dissolve  the  salt  in  dilute 
hydrochloric  acid  and  combine. 

68.  Almost  ninety  per  cent,  of  the  magnesium  present  is  at  once 
thrown   down   as   amorphous  magnesium   hydrogen  phosphate, 
MgHPO4. 

69.  The  addition  of  ammonia  and  the  stirring  transform  the 
magnesium  hydrogen  phosphate  into  crystalline  magnesium  ammo- 
nium   phosphate,   MgNHiPCU.      Simultaneously  the  remaining 
magnesium  is  almost  completely  thrown  down.     The  complete 
precipitation  is  effected  upon  standing  two  or  three  hours. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  260. 


GRAVIMETRIC  ANALYSIS  77 

Let  stand  two  or  three  hours  to  insure  complete  precipita- 
tion. 

Decant  the  liquid  through  the  filter. 

Wash  the  precipitate  three  times  by  decantation  Fatenng  and  wash- 
with  a  wash  water  made  by  mixing  one  a^n^riu^hos^1 
part  of  ammonia  (sp.  gr.  0.96)  and  five  parts  phate 
of  water. 

Transfer  to  the  filter  and  wash  till  the  filtrate  gives  with  silver 
nitrate  no  reaction  for  chlorids. 

Dry  in  the  hot  closet. 

Transfer  the  bulk  of  the  dried  precipitate  to  a  weighed  porcelain 
crucible. 

Burn  the  filter  paper  in  a  platinum  wire  spiral  and  add  the  ash 
to  the  precipitate  in  the  crucible. 

Heat  the  covered  crucible  very  gently  until  the  ammonia  is 
driven  off  and  the  precipitate  is  white.   Ignition,  cooling 
Then  heat 70  more  strongly  (see  reaction  X) .   and 

Cool  in  a  desiccator. 

Ignite  to  a  constant  weight.     Use  the  blast  lamp  if  necessary. 


EXPLANATORY  FACT 


70.  During  this  ignition,  great  care  should  be  used  to  avoid 
the  reduction  of  the  pyrophosphate. 


78  QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  SILICA,  SiO2, 

IN 
GLASS  OR  OTHER  INSOLUBLE  SILICATES 


The  silicate  is: 

(a)  converted  by  fusion  with  an  alkali  carbonate  into  a  form 
decomposable  by  hydrochloric  acid.  The  metals  are 
converted  to  carbonates  while  the  silicic  acid  forms  sili- 
cates of  potassium  and  sodium. 

(6)  dissolved  in  hydrochloric  acid  with  formation  of  chlorids 
and  free  silicic  acid. 

(c)  silicic  acid  is  dehydrated  to  silica,  SiO2. 

(d)  filtered,  ignited,  weighed  and  calculated  as  silicon  dioxid, 

Si02. 


REACTIONS 

I.  2M2"Si04  +  4NaKC03  =  4M 

or 
II.   2  M"Si03  +  2  NaKC03  =  2  M"C03  +  Na^SiOa  +  K2Si03. 

III.  M"CO3  +  2  HC1  =  MC12  +  H20  +  C02. 

IV.  Na4Si04  +  4  HC1  =  4  NaCl 

or 

V.   Na2Si03  +  2HCl  =  2NaCl 
VI.  EUSiO*  +  A  =  2  H2O  +  SiO2 

or 

VII.   H2SiO3  +  A  =  H2O  +  SiO2. 
VIII.  SiO2  +  4  HF  =  SiF4  +  2  H2O. 


GRAVIMETRIC  ANALYSIS  79 


DETERMINATION   OF  SILICA 


PROCEDURE 

Grind  the  mineral  in  an  agate  mortar  to  an  impalpable  powder. 
The  correctness  of  the  determination  de-  Grinding  and 
pends  upon  this  being  perfectly  done.  weighing  the  charge 

Weigh  into  platinum  crucibles  two  portions  of  the  substance, 
0.2  of  a  gram  each. 

Add  2  grams  of  sodium  potassium  carbonate,  NaKCOs1  to  each 
and  mix.     To  avoid  loss  by  frothing,72  the  Fusion  with  an 
crucible  should  never  be  filled  more  than  alkali  ""^nate 
one-half  full.    The  carbonate  can  be  added  in  small  portions 
during  the  fusion. 

Heat  gently  until  the  frothing  ceases.  When  melted,  heat  for 
fifteen  minutes  or  longer  over  a  blast  lamp  until  tranquil 
fusion  results. 


EXPLANATORY  FACTS 

71.  By  this  fusion  all  the  insoluble  substances  are  transformed 
into  such  compounds  as  carbonates,  silicates  of  the  alkalies,  etc., 
which  are  all  decomposed  by  hydrochloric  acid.     The  final  solu- 
tion, then,  is  composed  of  chlorids  of  those  metals  present  and 
silicic  acid.     According  to  the  amount  present,  the  silicic  acid  is 
wholly  or  in  part  in  solution. 

72.  The  frothing  is  caused  by  the  evolution  of  the  carbon 
dioxid  gas. 


80  QUANTITATIVE  ANALYSIS 

After  the  lamp  has  been  removed  and  when  the  crucible  is  just 
below  redness,  cool  suddenly 73  by  placing  it  on  an  inverted 
porcelain  mortar,  or  stone  desk- top. 

When  thoroughly  cooled,  invert  the  crucible  on  a  piece  of  glazed 
paper  and  remove  the  fused  cake. 

Place  the  crucible  with  its  adhering  pieces  of  the  fused  mass  and 
its  cover  hi  a  beaker. 

Add  100  c.c.  of  water,  cover,  warm  and  from  time  to  time 
add   a  little  dilute   hydrochloric   acid   till   Preparation  of  the 
all    of    the    adhering    substance    is    dis-   solution 
solved. 

Remove  the  crucible  and  its  cover  with  a  stirring  rod  and  care- 
fully wash  them  both  off  into  the  beaker. 

Put  the  fused  cake  from  the  glazed  paper  into  the  beaker  and 
dissolve  it  with  about  50  c.c.  of  hydrochloric  acid  (sp.  gr. 
1.12).  Keep  the  beaker  closely  covered  to  avoid  loss  from 
effervescence.74 

Transfer  the  acid  solution  to  an  evaporating  dish  or  casserole 
and  evaporate  to  dryness  on  a  steam  bath. 

Heat  in  a  hot  Closet  at   130°  C.  for  tWO  hours      Dehydration  of  the 

(see  reactions  VI  &  VII).  silicic  acid 

Cool    and    add    a    few    drops    of    concentrated    hydrochloric 

acid. 

Warm  cautiously.  To  the  warm  solution  add  20  c.c.  of  hydro- 
chloric acid  (sp.  gr.  1.12)  and  then  20  C.C.  Filtration  and  wash- 

of  water.     Again  warm  gently.  *"*  of  the  sUica 

While  warm,  filter  into  a  beaker. 


EXPLANATORY  FACTS 

73.  This  loosens  the  mass  from  the  side  of  the  crucible. 

74.  Silicic  acid,  which  is  slightly  soluble  in  water  and  in  acids, 
is  a  white  gelatinous  mass.    It  appears  yellow  as  it  is  suspended 
in  a  yellow  solution. 


GRAVIMETRIC  ANALYSIS  81 

Wash  the  residue  on  the  filter  paper  till  free  from  hydrochloric 

acid.  Ignition  and  weigh- 

Dry,  ignite  in  a  platinum  crucible  and  weigh.75     "« 


EXPLANATORY  FACT 

75.  After  a  constant  weight  has  been  obtained,  a  test  of  the 
purity  of  the  silica  consists  in  volatilizing  the  silica  with  hydro- 
fluoric acid,  HF  (see  reaction  VIII).  The  silicon  tetrafluorid 
formed  volatilizes,  while  compounds  of  any  admixed  metals,  if 
present,  remain  in  the  crucible.  Deduction  of  the  weight  of 
this  nonvolatile  residue  from  the  constant  weight  of  silica  ob- 
tained, gives  the  weight  of  the  pure  silica. 


SECTION  II 


ELECTROLYTIC   ANALYSIS 


83 


ELECTROLYTIC   ANALYSIS. 


lonization 

WHEN  most  compounds  are  dissolved,  their  molecules  are  dis- 
sociated, breaking  down  into  ions,  and  the  process  is  called  ioniza- 
tion  or  dissociation. 

Ions  are  electrically  charged  and  in  this  respect  differ  from  atoms 
or  molecules. 

A  molecule  of  common  salt,  NaCl,  is  composed  of  atoms  of 
sodium  and  chlorin,  each  of  which  has  its  definite  characteristics. 
When  these  atoms  of  sodium  and  chlorin  become  electrically 
charged  and  have  become  ions,  their  characteristics  have  changed. 
Metallic  sodium  decomposes  water  and  forms  sodium  hydroxid, 
but  if  this  same  sodium  becomes  ionized,  it  exists  in  water  with- 
out chemical  change.  When  this  ion  loses  its  electric  charge, 
it  immediately  decomposes  the  water,  forms  sodium  hydroxid  and 
liberates  hydrogen. 

That  these  ions  are  electrically  charged  can  be  demonstrated. 
Two  conducting  plates  acting  as  electrodes,  the  poles  of  an  open 
electric  circuit,  one  positive  and  the  other  negative,  are  immersed 
in  a  dilute  solution  of  common  salt.  Immediately  the  sodium  is 
attracted  to  the  negative  pole.  The  negative  pole  loses  some  of  its 
negative  charge  and,  it  is  supposed,  neutralizes  the  positive  charge 
of  the  sodium  ion,  which  assumes  the  atomic  state  and  shows 
the  characteristics  of  metallic  sodium  by  decomposing  water  to 
form  sodium  hydroxid.  The  chlorin  is  attracted  to  the  positive 

85 


86  QUANTITATIVE  ANALYSIS 

pole  which  loses  some  of  its  charge  of  positive  electricity,  presum- 
ably neutralizing  the  negative  charge  of  the  chlorin  ion.  The 
chlorin  escapes  at  the  positive  pole  and  has  all  the  characteristics 
of  chlorin  in  its  ordinary  state. 

A  solution  of  a  dissociated  substance  is  called  an  electrolyte.    The 
greater  the  dilution  the  more  complete  the  dissociation. 


Electrolysis 

The  process  of  decomposition  of  a  chemical  compound  in  solu- 
tion by  the  electric  current  is  called  electrolysis.  The  compound 
is  decomposed  into  two  parts,  each  of  which  may  be  simple,  like 
copper,  Cu,  or  complex,  like  SCX 

Hydrogen  or  metallic  ions  formed  in  solution  by  dissociation 
and  charged  with  plus  electricity  are  attracted  to  the  negative 
pole  or  cathode  and  are  therefore  called  cations. 

Nonmetallic  ions  formed  in  solution  by  dissociation  and  charged 
with  negative  electricity  are  attracted  to  the  positive  pole  or 
anode  and  are  therefore  called  anions. 

The  current  passes  through  the  liquid  between  the  two  metallic 
surfaces,  —  electrodes.  These  may  be  plates,  spirals,  cones,  etc., 
according  to  the  method  to  be  employed.  They  may  be  fixed  or 
arranged  to  rotate.  The  current  enters  by  the  positive  pole,  the 
anode,  and  leaves  by  the  negative  pole  of  the  circuit,  the  cathode. 
The  anions  separate  at  the  anode  and  the  cations  at  the  cathode. 

In  the  following  determination  the  metals  silver  and  copper 
are  cations  and  are  therefore  separated  at  and  deposited  on 
the  cathode.  The  acid  radical  of  the  nitrates  separates  at  the 
anode. 

Current  density  is  the  proportion  of  the  strength  of  the  current 
to  the  electrode  surface  and  is  usually  expressed  by  amperage  per 
100  sq.  cm.  In  the  potassium  cyanid  solution  used  here  (see  p.  88) 
the  approximate  current  density  used  for  silver  is  2.5  volts  and 
0.06-0.1  ampere  per  100  sq.  cm.  and  that  of  copper  is  5  volts  and 
1  ampere. 

Silver  is  thus  first  deposited  and  then  the  copper  is  determined 
in  the  remaining  solution  by  using  a  higher  current  density.  The 


ELECTROLYTIC  ANALYSIS  87 

metals  are  deposited  on  weighed  electrodes  and  the  per  cent,  deter- 
mined by  the  observed  increase  in  weight. 


Such  processes  save  time,  are  cleaner  and  lend  themselves  to 
easier  manipulation  than  is  the  case  with  ordinary  gravimetric 
methods.  The  use  of  electrolysis  for  analysis  is  rapidly  increas- 
ing. The  following  is  but  one  of  the  many  now  established  for 
the  separation  and  determination  of  metals  alone  or  hi  the  presence 
of  others. 


88  QUANTITATIVE  ANALYSIS 


THE  ELECTROLYTIC   SEPARATION* 

OF 
COPPER  AND   SILVER  f 

A  Type  of  Electro-Analysis 

(a)  The  alloy  is  dissolved  in  nitric  acid,  forming  nitrates  of  silver 

and  copper. 
(6)  The  excess  of  nitric  acid  is  removed  by  evaporation. 

(c)  An  excess  of  potassium  cyanid  is  added. 

(d)  With  proper  strength  of  current,  the  silver  is  deposited  on  a 

weighed  platinum  electrode. 

(e)  With  the  proper  strength  of  current,  the  copper  is  deposited 

on  a  similar  electrode. 


REACTIONS 

I.  Ag  +  2  HNO3  =  AgNO3  +  H2O  +  NO2. 

II.  3  Cu  +  8  HN03  =  3  Cu(N03)2  +  4  H20  +  2  NO. 

III.  AgN03  +  2  KCN  =  KAg(CN)2  +  KNO3. 

IV.  2  Cu(NO3)2  +  4  KCN  =  2  Cu(CN)2  +  4  KN03. 
V.  2  Cu(CN)2  =  (CN)2  +  2  CuCN. 

VI.  2  CuCN  +  6  KCN  =  2  (CuCN.3  KCN). 


*  This  particular  method  has  been  selected  because  of  its  simplicity.  The 
directions,  which  are  for  an  alloy  high  in  copper,  may  by  slight  modifications  be 
used  for  the  common  alloy,  coinage  silver  (see  Fact  76) . 

t  Smith  and  Frankel,  J.  A.  C.  S.,  1890,  p.  104;  Smith  and  Spencer,  J.  A.  C.  S., 
1894,  p.  420;  Smith  and  Fulweiler,  J.  A.  C.  S.,  1901,  p.  682;  Electro-Analysis,  Edgar 
F.  Smith,  1907. 


ELECTROLYTIC  ANALYSIS 


DETERMINATION  OF  SILVER  AND   COPPER 
PROCEDURE  * 

Weigh  one  portion  of  about  0.5  of  a  gram  of  sil- 
ver-copper alloy 76  into  a  250  c.c.  casserole. 
Read  Fact  77  and  then  dissolve  the  alloy  in  the 

Dissolving  the  alloy 

least  possible  amount  of  nitric  add   (1.4 

sp.  gr.). 

Evaporate  to  dryness  on  the  water  bath. 
While  this  is  on  the  water  bath,  clean  with  sapolio  and  then 

chromic  acid  the  platinum  electrode  that   preparation  of  a 

is  to  be  used  as  the  cathode.  cathode 

Wash  it  with  distilled  water  until  it  is  thoroughly  rinsed. 
Hold  it  in  the  pincers  and  pass  it  through  a  flame  till  it  is  red 

hot. 
Put  it  into  a  desiccator  to  cool. 


EXPLANATORY  FACTS 

76.  If  coinage  silver  —  ninety  parts  of  silver  to  ten  of  copper 
—  is  used  for  this  determination,  use  less  KCN  than  in  the  above 
directions,  allow  more  time  for  depositing  the  silver  and  less  for 
depositing  the  copper.    A  quarter  of  a  ten-cent  piece  weighs  about 
0.7  of  a  gram. 

77.  All  excess  of  nitric  acid  must  be  removed  in  the  next  step 
by  evaporation  to  dryness.     KCN  must  not  be  added  to  an  acid 
solution.     It  would  evolve  volatile  HCN,  which  is  a  poison. 

*  This  process  is  given  through  the  courtesy  of  Edgar  F.  Smith,  Professor  of 
Chemistry  at  the  University  of  Pennsylvania.  His  experiments  were  made  with  an 
alloy  of  approximately  ten  parts  of  silver  to  ninety  parts  of  copper. 


90  QUANTITATIVE  ANALYSIS 

When  cool,  weigh  and  note  the  weight. 

When  the  above  solution  is  evaporated  to  dryness,  dissolve  the 
residue  in  hot  water. 

Transfer  the  solution  quantitatively  to  a  100  c.c.  volumetric 
measuring  flask.* 

Wash  down  the  inside  of  the  neck  and  add  distilled  water  till 
the  meniscus  (see  page  99)  is  exactly  at  the  100  c.c.  line  on 
the  neck.  Dry  the  inside  walls  of  the  neck  above  the  line 
with  a  piece  of  filter  paper  rolled  around  a  stirring  rod. 

Pour  the  solution  back  and  forth  into  a  perfectly  dry  beaker  till 
it  is  thoroughly  mixed.  When  not  in  use  keep  this  meas- 
uring flask  closely  stoppered  to  prevent  evaporation. 

Measure  with  a  25  c.c.  volumetric  measuring  flask  or  pipet78 

exactly  25  c.c.  of  the  solution.     This  is  an   Measuring  an 
.   aliquot  part,  one-quarter  of  the  solution.   «"««<*  part 

Pour  this  aliquot  part  into  a  small  dry  beaker  holding  about 
200  c.c.  With  distilled  water  wash  out  several  times  the 
25  c.c.  flask  to  be  sure  that  the  solution  is  quantitatively 
transferred. 

Dilute  to  about  125  c.c. 

Read  Fact  79  and  then,  following  precautions,  add  about  2 
grams80  of  the  potassium  cyanid,  KCN.f 


EXPLANATORY  FACTS 

78.  Aliquot  parts,  or  definite  fractions  of  the  whole  volume  of 
a  uniform  solution,  should  be  measured  only  in  dry  measuring 
dishes  or  in  those  which  have  been  washed  out  with  several  small 
portions  of  the  solutions  to  be  measured. 

79.  Potassium  cyanid  is  a  violent  poison!    The  student  must 
handle  it  with  the  greatest  care!     He  must  wash  his  hands  care- 
fully after  using  it  and  at  no  time  during  this  experiment  should 
he  drink  from  laboratory  dishes. 

80.  The  amount  of  KCN  added  varies  according  to  the  relative 
per  cent,  of  the  copper  and  silver.     Use  approximately  2  grams  of 
KCN  for  each  0.1  of  a  gram  of  copper. 

*  Read  the  paragraph  on  "Measuring  Flasks,"  page  99. 

t  "Chemistry  of  the  Metals,"  Experiments  Nos.  49  and  104. 


ELECTROLYTIC  ANALYSIS  91 


DETERMINATION  OF  SILVER 

With  the  aid  of  the  instructor,  attach  the  weighed  cathode  to  the 
negative  pole  of  the  circuit  and  the  anode  to  the  positive 
pole. 

Arrange  the  two   electrodes  about  one   centimeter  apart  in 
the  solution.     Do  not  let  them  touch  the   Electrolytic 
bottom  of  the  beaker.  deposition 

Use  a  current  density  of  approximately  2.5  volts  and  0.06  to  0.1 
of  an  ampere  for  100  sq.  cm.  of  cathode  surface.81 

Heat  the  solution  to  a  temperature 82  of  about  65°  C. 

Test  with  a  thermometer  but,  to  avoid  loss  when  it  is  taken  out, 
wash  it  off  into  the  solution. 

Switch  on  the  electric  current  for  the  deposition  of  the  silver  and 
allow  from  two  to  three  hours. 

At  the  end  of  two  hours  one  drop  of  the  solution  Test  for  complete 
may  be  tested  for  silver.  deposition  of  silver 

When  there  is  no  silver  left  in  the  solution  and  the  deposition 
is  therefore  shown  to  be  completed,  have  at  hand  bottles  of 
alcohol  and  ether,  a  wash  bottle  and  a  piece  of  filter  paper 
laid  on  a  five-inch  watch  glass.  The  following  work  should 
be  done  expeditiously. 


EXPLANATORY  FACTS 

81.  Unless  connections  have  been  used  in  previous  work  and 
the  terminals  marked,  it  will  be  necessary  to  determine  before- 
hand by  experiment  which  is  the  negative  pole.    This  can  be  done 
by  the  use  of  an  ammeter  upon  which  the  poles  are  marked  or  by 
passing  a  current  through  a  dilute  copper  sulfate  solution  in  which 
there  are  two  unweighed  platinum  electrodes.     The  copper  is  de- 
posited on  the  negative  electrode. 

82.  At  a  temperature  of  65°  C.,  0.2  to  0.3  of  a  gram  of  silver 
may  be  precipitated  in  four  hours.     If  the  solution  is  cold,  it  will 
probably  take  ten  hours  for  0.2  of  a  gram  to  precipitate. 


92  QUANTITATIVE  ANALYSIS 

Lift  the  cathode  from  the  solution. 

Wash  it  thoroughly  with  a  stream  of  water   Preparation  of  the 

from  the  wash  bottle,  catching  the  wash- 

ings  in  the  beaker.83 
Pour  alcohol M  completely  over  each  side  of  the  electrode. 
Do  the  same  with  ether. 

Place  the  cathode  on  the  filter  paper  on  the  watch  glass. 
Put  the  watch  glass  in  an  air  bath  that  is  not  hotter  than  100°  C., 

for  a  few  moments  only,  until  dry.     Lest  the  cathode  be  for- 
gotten, do  nothing  else  until  it  is  dry. 
Place  in  a  desiccator,  cool  and  weigh.     Note  the  weight. 
After  the  final  weight  has  been  made,  in  a  beaker  dissolve  the 

silver  from  the  cathode  with  hot,  dilute  nitric  add. 
Put  this  solution  in  the  "silver  residue"  bottle.     Wash  the 

cathode,  polish  it  with  sapolio,  clean  it  thoroughly,  dry, 

ignite  and  weigh. 
Leave  it  in  the  desiccator  ready  for  the  next  deposition. 


DETERMINATION  OF  COPPER 

The  silver  has  been  removed  but  the  solution  still  contains  the 
copper  which  did  not  deposit  at  so  low  a  current  density.  The 
potassium  cyanid  which  was  added  at  the  start  is  still  present. 
The  best  working  volume  under  these  conditions  has  been  found 
to  be  150  c.c. 

This  solution  is  probably,  because  of  previous  washings,  about 
150  c.c.  If  not,  evaporate  it  to  this  volume. 


EXPLANATORY  FACTS 

83.  The  solution  that  clings  to  the  cathode  contains  some  of 
the  copper  which  is  to  be  quantitatively  determined. 

84.  Do   not   pour   alcohol    and   ether   near   a   flame.     Allow 
"washings"   to  fall  into  a  clean,  dry  beaker  to  be  saved   in 
"residue  bottles." 


ELECTROLYTIC  ANALYSIS  93 


PROCEDURE 

Add  10  c.c.  of  ammonium  hydroxid^  NH4OH  (0.96  sp.  gr.). 

Arrange  the  electrodes  as  before. 

Use  a  current  density  of  approximately  5  volts  and  1  ampere 

for  100  sq.  cm. 

Keep  the  temperature  of  the  solution  at  65°  C. 
At  65°  C.,  about  0.2  of  a  gram  of  copper  will   Deposition  of  the 

be  deposited  in  an  hour.  c°pper 

Two  hours  therefore  should  be  allowed.     At  the  end  of  this  time, 

add  a  little  distilled  water  to  the  solution  to  raise  its  level. 
If  no  new  copper  appears  on  the  fresh  surface  of  the  platinum, 

the  deposition  may  be  considered  complete. 
Lift  the  cathode  from  the  solution  and  wash  quickly  and  thor- 
oughly with  distilled  water. 
Wash  with  alcohol  and  with  ether  as  before,   Preparation  of  the 

save  washings  in  the  "residue  bottles." 
Place  the  cathode  on  a  filter  paper  on  a  watch 

glass  and  put  in  an  air  bath  not  hotter  than  100°  C.  till  dry, 

but  no  longer. 

Cool  in  a  desiccator  and  weigh. 
After  the  weight  has  been  made,  dissolve  the  copper  from  the 

cathode  with  dilute  nitric  acid. 
Wash  the  cathode  and  as  previously  described  prepare  it  for  the 

check  analysis. 


EXPLANATORY  FACT 

85.  In  alkaline  cyanid  solution,  there  is  a  tendency  for  the 
platinum  of  the  anode  to  be  deposited  with  the  copper  on  the 
cathode.  If,  however,  the  cyanid  is  not  largely  in  excess,  and  if 
the  current  is  interrupted  as  quickly  as  possible  after  the  copper 
is  deposited,  this  error  is  reduced  to  a  minimum.  It  has  been 
determined  that  in  the  presence  of  a  definite  amount  of  ammonium 
hydroxid  there  is  absolutely  no  loss  sustained  by  the  anode  in 
cyanid  electrolyte  and  that  the  precipitation  is  much  acceler- 
ated ("Electro- Analysis,"  Smith,  page  71). 


94  QUANTITATIVE  ANALYSIS 


Duplicate  Analysis 

Use  another  aliquot  part  of  25  c.c.  of  the  original  solution  and 
make  a  check  analysis  of  both  the  silver  and  copper. 


SECTION  III 


VOLUMETRIC  ANALYSIS 


95 


VOLUMETRIC   ANALYSIS 


Comparison  of  Volumetric  and  Gravimetric  Methods 

IN  Gravimetric  Analysis  the  element  or  radical  to  be  determined  is 
either  isolated  or  combined  in  an  insoluble  compound  and  weighed. 
This  process  of  isolation  or  combination  often  requires  a  series  of 
careful  and  prolonged  operations.  In  Volumetric  Analysis  (1)  the 
element  or  radical  to  be  determined  is  not  necessarily  isolated  but 
is  often  treated  in  the  presence  of  the  other  constituents.  To  make 
this  possible,  the  exact  qualitative  content  should  be  known  and 
all  reactions  that  are  likely  to  occur  in  the  course  of  the  analysis 
should  be  understood.  (2)  In  the  oftentimes  greater  simplicity  of 
the  processes  involved,  in  the  fewer  weighings,  and  in  the  usual 
absence  of  much  filtering  and  washing,  the  chance  of  error  is  less 
than  in  Gravimetric  Analysis  and  (3)  the  saving  of  time,  especially 
in  a  case  of  routine  work  which  requires  the  analysis  of  many 
samples  a  day,  is  very  great. 

Volumetric  Analysis  is  the  quantitative  determination  of  an 
element  or  radical  in  a  substance,  —  a  determination,  as  its  name 
indicates,  made  by  adding  a  volume  of  a  selected  solution  sufficient 
to  cause  definite,  complete  reaction. 


97 


98  QUANTITATIVE  ANALYSIS 


Measuring  Instruments 

For  the  accurate  measurement  of  liquids  there  are  certain 
graduated  glass  vessels,  the  pipet,  measuring  cylinder,  measuring 
flask  and  buret. 

For  the  best  results,  it  is  most  important  that  the  measuring 
instruments  be  graduated  accurately  and  that  they  agree  among 
themselves. 

In  a  longer  course  of  Volumetric  Analysis  it  is  customary 
to  have  the  students  calibrate*  to  verify  the  capacity  of  these 
instruments.  In  this  short  course,  if  possible,  it  would  be  well 
for  the  department  to  have  the  instruments  calibrated  and 
to  issue  calibration  cards  showing  the  variation  of  the  instru- 
ments. If  this  is  not  possible  the  errors,  which  are  often  slight, 
may  be  ignored  and  a  wider  latitude  allowed  in  the  results 
obtained. 


Pipets  are  used  to  measure  exactly  small  amounts  of  solutions. 
They  are  either  graduated  to  deliver  a  specified  quantity,  such  as 
10  c.c.,  50  c.c.,  etc.,  or  are  graduated  in  fifths  or  tenths  of  a  cubic 
centimeter  to  deliver  any  desired  fractional  part  of  the  whole 
quantity.  The  pointed  end  of  the  pipet  is  dipped  into  the 
solution,  the  mouth  is  applied  to  the  other  end  and  the  liquid 
sucked  into  the  pipet  up  to  the  mark.  The  forefinger  is  placed 
over  the  top  end  to  keep  the  solution  from  running  out.  The 
pressure  of  the  finger  regulates  the  flow  from  the  pipet.  In  case 
of  a  "  one-quantity  "  pipet,  to  insure  complete  delivery,  the  pipet 
should  be  inclined  against  the  side  of  the  dish  and  the  last  drop  fi  ^ 
blown  out.  Pipets  are  graduated  to  deliver  their  contents;  the 
mark  is  therefore  high  enough  to  include  the  liquid  that  adheres  to 

*  For  methods  of  calibrating  pipets,  flasks,  cylinders  and  burets,  see  Button's 
"Volumetric  Analysis,"  pages  18-20. 


•'    VOLUMETRIC  ANALYSIS  99 

the  inside  walls;  in  other  words,  it  delivers  into  another  receptacle 
the  exact  amount  marked,  but  holds  a  trifle  more.  The  point  of 
a  pipet  should  be  sufficiently  fine  so  that  the  liquid  will  not  be 
delivered  too  quickly. 


Measuring  flasks,  stoppered  glass  vessels  with  narrow  necks,  are 
graduated  either  to  deliver  or  to  contain  or  both,  in  which  latter 
case  the  upper  mark  is  that  of  delivery.  Each  measuring  flask  and 
pipet  should  have  etched  upon  it  its  capacity,  the  temperature 
at  which  the  graduation  was  made  and  the  line  that  marks  the 
limit  of  definite  content.  A  specially  constructed  flask  has  along 
the  length  of  the  inside  wall  of  its  neck  a  broad  blue  line  on 
a  background  of  white  enamel.  This  facilitates  the  reading  by 
the  reflection  of  the  blue  on  the  "meniscus"  —  the  curved  line 
of  the  surface  of  the  liquid  —  which  makes  the  blue  line  narrow 
into  a  point. 


Measuring  cylinders,  glass  cylinders  of  varying  sizes,  are  gradu- 
ated to  deliver  and  are  used  for  mixing  and  for  making  compara- 
tively rough  measurements. 


Burets,  graduated  glass  tubes  of  large  bore,  with  cocks  at  the 
lower  end  for  controlled  delivery,  are  graduated  in  fifths,  tenths 
or  twentieths  of  a  cubic  centimeter  and  hold  twenty-five,  fifty  or 
one  hundred  cubic  centimeters.  The  two  common  forms  of  buret 
are  the  Geissler,  or  Fresenius,  type  with  glass  stopcock  and  the 
Mohr  type  with  a  rubber  tip  closed  with  a  metal  pinchcock  or  a 
glass  ball  acting  as  a  valve.  Burets  are  supported  upon  stands. 
One  convenient  model  is  the  Chaddock,  in  which  the  burets  are 
held  by  easily  manipulated  wire  clips  which  prevent  slipping  and 
hold  the  buret  firmly  vertical.  The  white  porcelain  base  serves 
as  an  excellent  background  against  which  end  points  are  well 
determined. 


100  QUANTITATIVE  ANALYSIS 


Reading  the  Instruments 

The  surface  of  liquids  in  narrow  vessels  is,  because  of  capillary 
attraction,  concave  and,  in  the  case  of  mercury,  convex.  This 
makes  it  necessary  to  select  some  point  on  the  curved  line  to 
coincide  with  the  graduation  mark  in  the  instrument.  In  the 
case  of  all  light-colored  liquids,  it  has  been  found  advisable  to 
select  the  lowest  point  of  the  concave  line,  —  the  bottom  point 
of  the  "meniscus";  with  dark  or  opaque  liquids,  the  highest  point 
on  the  extreme  upper  level. 

Before  reading  any  graduated  instrument,  its  exact  vertical  posi- 
tion must  be  assured. 

In  reading  a  buret,  the  eye  should  be  on  a  level  with  the  selected 
point  of  the  meniscus  and  its  position  in  relation  to  the  graduated 
lines  on  the  buret  noted. 

If  the  student  is  not  provided  with  a  buret  made  with  a  longi- 
tudinal blue  line  on  a  white,  enameled  background,  he  may,  with 
advantage,  hold  a  piece  of  dark  glazed  paper  at  the  back  of  the 
buret  about  one-eighth  of  an  inch  below  the  surface  of  the  liquid. 
This  will  make  the  lower  part  of  the  meniscus  appear  as  a  well- 
defined  black  line  against  the  white. 

A  buret  reading  should  not  be  taken  until  all  liquid  which  may 
cling  to  the  walls  of  the  buret  has  had  time  to  flow  into  the  main 
bulk  of  the  liquid.  Three  minutes  is  the  usual  time  allowed  for 
the  liquid  to  collect.  A  uniform  rate  of  delivering  the  liquid  should 
be  adopted.  After  a  rapid  run  it  takes  more  time  for  the  liquid  to 
collect  than  after  a  slow  run. 


VOLUMETRIC  ANALYSIS  101 


General  Directions 

The  perfect  cleanliness  of  the  volumetric  measuring  apparatus 
is  essential.  Grease,  which  makes  the  liquid  gather  in  drops  on  the 
walls,  may  be  removed  with  a  dilute  solution  of  sodium  hydroxid, 
NaOH;  or  acidified  potassium  dichromate,  K2Cr2O7,  or  chromic 
acid,  CrO3. 

Any  volumetric  instrument  even  if  new  and  dry  should  be  rinsed 
out  with  three  successive  small  portions  of  the  solution  with  which 
it  is  to  be  filled.  It  is  absolutely  necessary  that  all  standard  solu- 
tions should  be  kept  at  their  known  established  strength,  that  they 
should  not  be  diluted  with  any  drops  of  water  which  may  be 
adhering  to  the  walls  of  the  measuring  vessels  and  that  they  should 
not  be  contaminated  with  any  foreign  matter. 

In  Volumetric  Analysis  to  insure  accurate  results  the  conditions 
should  be  kept  as  uniform  as  possible,  —  conditions  of  intensity 
of  light,  temperature,  time  of  waiting  before  taking  readings, 
reading  of  tenths  and  end  points.  To  practise  the  reading  of 
tenths,  the  buret  may  be  filled  with  water  and,  as  it  is  allowed  to 
escape  drop  by  drop,  the  surface  of  the  water,  as  the  level  is 
lowered,  may  be  watched  and  its  exact  position  noted  in  reference 
to  the  graduation  lines.  In  this  way,  accurate  reading  of  the 
tenths  is  easily  mastered. 


102  QUANTITATIVE  ANALYSIS 


Standard  Solutions 

A  solution  whose  exact  content  per  cubic  centimeter  has  been 
determined  is  a  standard  solution  and  is  said  to  have  been  stand- 
ardized. 

The  process  of  adding  a  definite  quantity  of  a  standardized 
liquid  to  a  solution  to  estimate  its  value  or  to  a  solution  of  a  sub- 
stance to  be  analyzed  is  called  titration. 

An  acid  solution,  for  instance,  is  standardized  and  alkali  titrated 
with  it  to  estimate  the  value  of  the  alkali  in  terms  of  the  acid  and 
vice  versa.  A  solution  of  a  salt  is  standardized  to  be  used  for  the 
oxidation,  reduction  or  precipitation  of  the  solution  to  be  analyzed. 
'A  solution  of  known  composition  and  standard  strength  is  used 
to  cause  a  complete,  definite  reaction  with  a  solution  of  the  sub- 
stance to  be  analyzed.  For  example,  a  solution  of  ammonium 
sulfo-cyanid  whose  exact  strength  is  known  is  added  to  a  solution 
of  silver  nitrate  to  be  analyzed.  These  react  as  follows: 

AgN03  +  NH4SCN  =  AgSCN  +  NH^Oa. 

When  the  silver  in  the  silver  nitrate  is  entirely  changed  to  sulfo- 
cyanid,  another  drop  of  the  ammonium  sulfo-cyanid  will  form  no 
further  precipitate  (see  under  "  End  Point"  2,  b).  The  strength 
of  the  ammonium  sulfo-cyanid  is  known  and  also  the  exact  amount 
in  cubic  centimeters  that  it  took  to  precipitate  the  silver.  From 
this  data  and  the  relation  of  the  molecular  weights  the  amount  of 
silver  present  in  the  silver  nitrate  can  be  calculated. 


VOLUMETRIC  ANALYSIS  103 


End  Point 

The  exact  point  at  which  such  reactions  are  complete  is  called 
the  end  point.  The  end  point  is  shown: 

1.  By  the  persistence  of  the  color  of  the  standard  solution  used. 
This  shows  the  completion  of  chemical  action.     Thus:  A  solution 
of  ferrous  sulfate  is  titrated  with  a  standardized  solution  of  potas- 
sium permanganate  in  the  presence  of  sulfuric  acid, 

2  KMn04  +  10  FeSO4  +  8  H2S04 

=  2  MnSO4  +  5  Fe2(S04)3  +  K2SO4  +  8  H20. 

The  color  of  the  standard  solution,  KMnO4,  is  pink.  Just  as 
long  as  any  of  the  ferrous  sulfate  is  left,  the  potassium  perman- 
ganate continues  to  decompose  and  to  lose  its  pink  color.  When 
the  reaction  is  completed,  the  permanganate  is  no  longer  decom- 
posed and  its  pink  color  persists. 

2.  By  the  formation  of,  or  change  in,  the  color  of  a  substance, 
called  an  indicator,  originally  added  for  this  purpose  to  the  solution 
to  be  analyzed.     Thus: 

a.  With  an  organic  indicator,  such  as  methyl  orange,  in  the 
presence  of  alkalies  its  red  color  changes  to  yellow  or,  again, 
colorless  phenolphthalein  in  the  presence  of  alkalies  turns  bright 
red. 

6.  With  an  inorganic  indicator,  such  as  ferric  chlorid  in  the 
following  illustration,  after  the  main  reaction  is  completed,  the 
soluble,  red  Fe(SCN)3,  is  formed. 

NH4N03 
3NH4C1 


3.  By  the  formation  of  a  precipitate.     Thus:  A  standardized 
solution  of  silver  nitrate  is  added  to  a  solution  of  potassium 


AgN03      + 
Substance  to 
be  analyzed 

NH4SCN        = 
Standard 
solution 

AgSCN 

White 
precipitate 

FeCl3        + 
Soluble,  color  not 
noticed  when  dilute 

3NH4SCN    = 

First  drop 
in  excess 

Fe(SCN)3 
Soluble, 
red 

104  QUANTITATIVE  ANALYSIS 

chlorid  to  be  analyzed,  in  which  there  is  an  indicator,  potassium 
chromate. 

KC1  +  AgN03  =  AgCl  +  KN03. 

When  all  of  the  potassium  chlorid  has  been  used  to  form  white 
silver  chlorid,  the  next  drop  of  the  reagent,  silver  nitrate,  reacts 
with  the  potassium  dichromate  to  form  silver  chromate  which  is 
red. 

K2CrO4  +  2  AgNO3  =  Ag2CrO4  +  2  KNO3. 
Indicator  Red 

4.  By  the  failure  to  produce  color  when  a  drop  of  the  solution 
being  analyzed  is  added  to  an  indicator  in  a  separate  container. 
Thus:  A  standardized  solution  of  potassium  dichromate  is  added 
to  a  solution  of  ferrous  chlorid  to  be  analyzed  in  the  presence  of 
hydrochloric  acid, 

K2O2O7  +  6  FeCl2  +  14  HC1 

=  6  FeCl3  +  2  KC1  +  2  CrCl3  +  7  H2O. 

The  indicator,  potassium  ferricyanid,  is  in  a  separate  dish.  If, 
before  the  reaction  is  completed,  a  stirring  rod  is  dipped  into  the 
above  solution  and  then  into  the  indicator,  there  is  formed  a  blue 
precipitate,  according  to  the  equation, 

3  FeCl2  +  2  K3Fe(CN)6  =  Fe3(Fe(CN)6)2  +  6  KC1. 
Indicator  Blue 

When  the  ferrous  chlorid  is  all  oxidized  to  ferric  chlorid  and  a  drop 
of  the  solution  is  added  to  the  indicator,  there  will  be  no  color, 
according  to  the  equation, 


FeCl3  +  K3Fe(CN)6  =  FeFe(CN)6  +  3  KCL 
Indicator  Colorless 


VOLUMETRIC  ANALYSIS  105 


Indicators 

Indicators  as  illustrated  above  are  used  to  show,  by  color  changes 
or  by  a  precipitation,  a  condition  of  acidity,  alkalinity,  neutrality 
or,  in  general,  the  completion  of  reaction.  They  may  be  (1)  dye- 
stuffs;  (2)  the  coloring  matters  of  plants,  which  are  generally  weak 
acids;  salts  of  weak  acids  or,  less  often,  weak  bases;  or  (3)  in- 
organic compounds  which  form  a  coloration  or  a  colored  precipi- 
tate at  the  end  of  a  reaction. 

An  indicator  must  take  no  part  in  the  reaction  going  on  but  as 
the  reagent  is  added  must  itself  remain  inert  till  the  main  reaction 
is  completed.  After  the  completion  of  the  reaction,  the  next  drop 
of  the  liquid  acts  on  the  indicator  in  the  solution  and  gives  ocular 
evidence  that  the  end  point  has  been  reached.  There  is  no  one 
indicator  that  can  be  universally  used.  That  which  answers  for 
one  analysis  may  be  useless  for  another.  Its  value  depends  upon 
the  sensitivity  of  its  action  and  upon  the  sharpness  and  intensity 
of  the  color  it  affords. 

Indicators  have  been  divided  by  R.  T.  Thomson*  into  three 
classes :  (1)  Those  capable  of  forming  stable  salts,  as  methyl  orange. 
These  are  most  sensitive  to  alkalies.  (2)  Those  forming  unstable 
salts  which  hydrolyze  hi  water,  as  phenolphthalein.  These  are 
most  sensitive  to  acids.  (3)  Those  midway  between  the  two,  as 
litmus.  These  are  fairly  sensitive  to  both  alkalies  and  acids. 

The  theories  of  the  action  of  indicators  are  by  no  means  final. 
According  to  Ostwald,  an  indicator,  for  at  least  acids  and  bases, 
must  possess  a  color  when  undissociated  different  from  that  which 
it  has  when  ionized. f  That  is,  the  color  changes  are  ascribed  to 
the  dissociation  and  caused  by  the  addition  of  a  substance  to 
the  indicator.  It  is,  therefore,  the  color  reaction  caused  by  the 
hydrogen  or  hydroxyl  ions  of  dissociated  acids  or  bases  which 
makes  some  dyes  of  service  as  indicators. 

*  J.  S.  C.  I.,  volume  6,  1887,  page  195. 

t  Ostwald's  "Lehrbuch  der  Allgemeinen  Chemie." 


106  QUANTITATIVE  ANALYSIS 

Strong  acids  and  strong  bases,  since  they  ionize  readily,  are  not 
useful  as  indicators.  Weak  acids  and  weak  bases  are  more  or  less 
undissociated  in  solution  and  are  ionized  only  after  conversion  to 
a  neutral  salt. 

This  will  be  made  clear  by  the  following  examples : 

(a)  Phenolphthalein,  —  belonging  to  class  2,  —  a  weakly  acid 
indicator,  is  undissociated  and  colorless  in  acid  solution.  Its 
alkali  salt  is,  however,  at  once  dissociated  and  the  red  color  of 
the  ion  at  once  shows  the  completion  of  the  neutralization  of 
the  solution. 

(6)  Methyl  Orange,  —  belonging  to  class  1,  —  a  moderately 
strong  acid  indicator,  is  undissociated  and  of  a  pinkish  red  color 
in  acid  solution.  When  an  acid  solution  containing  the  indicator 
methyl  orange  is  titrated  with  a  solution  of  an  alkali,  the  first 
drop  of  alkali  after  the  end  point  has  been  reached  forms  a  salt 
with  the  methyl  orange.  The  salts  of  methyl  orange  are  readily 
dissociated,  whereupon  the  red  of  the  nonionized  indicator  changes 
to  yellow,  the  color  of  the  ion.* 

The  following  are  but  a  few  of  the  indicators  in  general  use: 
litmus,  methyl  orange,  phenolphthalein  and  lacmoid. 

Litmus 

With  acids red. 

With  alkalies blue. 

This  is  a  violet-blue,  organic  pigment  sensitive  to  both  acids 
and  alkalies.  It  cannot  be  used  with  bicarbonates  and  carbon- 
ates, as  the  liberated  carbonic  anhydrid  dissolves  and  causes  the 
red  color  to  persist  even  if  the  liquid  is  alkaline.  It  can,  however, 
be  used  if  the  solution  is  boiling,  as  this  gas  is  then  constantly 
evolved. 

Methyl  Orange 

With  acids pinkish  red. 

With  alkalies yellow.    ' 

This  is  the  sodium  or  ammonium  salt  of  a  specific  organic  acid, 
synthetically  prepared.  It  is  an  ochre  yellow  powder  which  when 

*  For  further  study  of  this  subject,  the  student  is  referred  to  such  textbooks  as: 
Jones's  "Theory  of  Electrolytic  Dissociation"  and  Cohn's  "Indicators  and  Test 
Papers,"  from  which  some  of  the  above  statements  are  taken. 


VOLUMETRIC  ANALYSIS  107 

dissolved  in  water  forms  a  yellow  solution.  Methyl  Orange  is 
particularly  sensitive  to  alkalies.  An  excess  of  this  indicator 
should  be  avoided,  as  the  color  changes  are  then  not  so  sharp. 
The  yellow  color  is  not  affected  by  alkalies  or  by  carbonates  of 
the  alkalies,  but  is  changed  to  a  pinkish  red  by  acids,  becoming 
yellowish  red  on  approaching  neutralization. 

Carbonic,  boric,  silicic,  arsenious  and  some  organic  acids,  in- 
cluding some  of  the  fatty  acids,  do  not  cause  methyl  orange  to 
change  to  red. 

Phenolphthalein 

With  acids colorless. 

With  alkalies red. 

This  is  an  organic  compound  synthetically  prepared.  It  is  a 
cream-white,  crystalline  powder.  Phenolphthalein  has  the  prop- 
perties  of  a  very  weak  acid  and  is  extremely  sensitive.  Even 
carbonic  acid  causes  a  change.  Titrations,  therefore,  in  the 
presence  of  carbonates,  can  be  done  only  in  boiling  solution.  One 
part  of  alkali  in  100,000  parts  of  water  will  give  a  distinct  color. 
It  cannot  be  used  in  the  presence  of  ammonium  salts. 

Lacmoid 

With  acids red. 

With  alkalies violet  blue. 

This  is  another  organic  compound  synthetically  prepared.  It 
is  in  the  form  of  blue-black  scales.  Lacmoid  is  sensitive  to  both 
acids  and  alkalies.  It  can  be  used  in  the  presence  of  carbonates 
only  in  hot  solution. 


108  QUANTITATIVE  ANALYSIS 


Normal  Solutions 


/N\ 
A  normal  solution ( yj is  one  that  contains  in  one  liter  as  many 

grams  of  the  active  substance  as  its  molecular  weight  divided 
either  by  its  replaceable  hydrogen  atoms  or  the  equivalent  of  such 
hydrogen  atoms. 


(A)  Reagents    with    replaceable    hydrogen    atoms,    such    as 
HC1,  H2S04. 

1.  The  molecular  weight  of  HC1  =  36.45.    This  expressed 

in  grams  is  36.45  grams.  HC1  has  one  replaceable 
hydrogen  atom;  therefore,  36.45  -*•  1  =  36.45,  the 
weight  required. 

A  normal  solution  of  HC1  contains  in  one  liter  36.45 
grams  of  HC1. 

2.  The   molecular  weight  of  H2SO4  =  98.07.     This   ex- 

pressed in  grams  is  98.07  grams.  H2S04  has  two 
replaceable  hydrogen  atoms;  therefore,  98.07  -4-  2 
=  49.03,  the  weight  required. 

A  normal  solution  of  H2S04  contains  in  one  liter  49.03 
grams  of  H2SO4. 

(B)  Reagents  containing  no  replaceable  hydrogen  atoms,  such 
as  NaOH,  K2Cr2O7  and  SnCl2. 

3.  The  molecular  weight  of  NaOH  is  40.058.     This  ex- 

pressed in  grams  is  40.058  grams.  Na  is  monovalent 
and  therefore  equivalent  to  one  atom  of  replaceable 
hydrogen. 

A  normal  solution  of  NaOH,  therefore,  contains  in  one 
liter  40.058  grams  of  NaOH. 


VOLUMETRIC  ANALYSIS  109 

The  equivalent  of  the  replaceable  hydrogen  atoms  differs  accord- 
ing to  the  reaction  that  is  to  take  place.  Potassium  dichromate, 
for  example,  can  be  used  either  as  a  precipitating  or  as  an  oxidiz- 
ing agent. 

4.   As  a  precipitating  agent,  potassium  dichromate  reacts 
as  follows: 

2  Ba(C2H3O2)2  +  K2Cr2O7  +  H2O 

=  2  BaCr04  +  2  KC2H302  +  2  HC2H302. 

In  this  reaction,  for  every  molecule  of  the  reagent  K2Cr207,  two 
molecules  of  BaCrO4  are  precipitated,  which  require  two  atoms 
of  Ba  in  the  two  molecules  of  Ba(C2H3O2)2.  Each  atom  of  Ba 
is  equivalent  to  two  of  H,  therefore  the  two  atoms  of  Ba  used  in 
this  precipitation  are  equivalent  to  four  atoms  of  hydrogen. 

In  a  normal  solution  of  potassium  dichromate  used  as  a  pre- 
cipitant, then,  its  equivalent  in  replaceable  hydrogen  atoms  is  four. 

The  molecular  weight  of  K2Cr2O7  =  294.5. 
This  expressed  in  grams  is  294.5  grams. 
294.5  +  4  =  73.62. 

A  normal  solution  of  potassium  dichromate  used  as  a  precipitant 
contains  in  one  liter  73.62  grams. 

5.   As  an  oxidizing  agent,  potassium  dichromate  reacts 
as  follows:,,  ^ 


*  6  FeCl2  +  K2Cr?07  +  14  HC1  = 

6  FeCl3  +  2  CrCl3  +  2  KC1  +  7  H20. 

In  this  process  of  oxidation,  K2Cr2O7  acts  as  if  it  split  up  into 
K20,  Cr203  and  O3.  Each  molecule  of  K2Cr207  frees  three  atoms 
of  O  for  oxidizing  purposes.  In  the  above  reaction,  for  three 
atoms  of  O  liberated  in  a  molecule  of  the  reagent  K2Cr2O7, 
there  are  six  atoms  of  chlorin  liberated  to  combine  with  the 
ferrous  chlorid  to  form  ferric  chlorid.  Six  atoms  of  Cl  are 
equivalent  to  six  atoms  of  H. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  200. 


110  QUANTITATIVE  ANALYSIS 

In  a  normal  solution  of  potassium  dichromate  used  as  an  oxidiz- 
ing agent,  its  equivalent  in  replaceable  hydrogen  atoms  is  six. 

The  molecular  weight  of  K2Cr207  =  294.5. 
This  expressed  in  grams  =  294.5  grams. 
294.5  grams  -J-  6  =  49.08  grams. 

A  normal  solution  of  potassium  dichromate  used  as  an  oxidizing 
agent  contains  in  one  liter  49.08  grams,  one-sixth  of  its  molecular 
weight  in  grams. 

6.   As  a  reducing  agent  stannous  chlorid  reacts  as  follows : 
SnCl2  +  2  FeClg  =  2  FeCl2  +  Sn<Cl4. 

In  this  reaction,  every  molecule  of  the  reagent,  SnCl2,  reduces 
two  molecules  of  the  ferric  chlorid  (2  FeCls)  to  two  molecules  of 
ferrous  chlorid  (2  FeCl2).  Each  molecule  of  the  SnCl2,  then, 
removes  two  atoms  of  chlorin  which  are  the  equivalent  of  two 
atoms  of  hydrogen.  This  may  also  be  shown  by  the  reaction 

<*•        V  In 

H2  +  2  FeCl3  =  2  FeCl2  +  2  HC1. 

In  a  normal  solution  of  stannous  chlorid  used  as  a  reducing  agent, 
its  equivalent  in  hydrogen  atoms  is  two. 

The  molecular  weight  of  SnCl2  =  189.90. 

This  number  expressed  in  grams  =  189.90  grams. 

189.90  -5-  2  =  94.95  grams. 

A  normal  solution  of  the  reducing  agent  stannous  chlorid  contains 
in  one  liter  94.95  grams  of  SnCl2,  one-half  of  its  molecular  weight  in 
grams. 

A  solution  of  one-half  the  strength  of  a  normal  solution  is  called 
a  seminormal  solution  ( -^-j  • 

A  solution  of  one  tenth  of  the  strength  of  a  normal  solution  is 

(N\ 
To)'  etc' 


VOLUMETRIC  ANALYSIS  111 


Permanency  of  Standard  Solutions 

Solutions  of  the  nature  of  those  just  described  do  not  remain 
of  the  same  strength  for  an  indefinite  period  of  time.  Evapora- 
tion and  chemical  changes,  including  those  resulting  from  the  action 
of  light,  and  action  upon  the  glass  of  the  bottle  all  contribute  to 
render  a  standardization  accurate  for  a  comparatively  limited 
time  only.  Thus,  silver  nitrate  is  reduced  in  the  light  and  silver 
is  deposited,  weakening  the  solution;  potassium  permanganate 
deposits  oxids  of  manganese,  and  sodium  hydroxid  reacts  with 
the  glass  of  the  bottle.  Even  at  normal  temperature,  a  solution 
of  any  substance  evaporates  and  the  solvent  condenses  on  the 
sides  of  the  bottle  or,  if  imperfectly  corked,  it  escapes  into  the  air. 
It  is  therefore  necessary  to  keep  the  bottle  carefully  stoppered,  to 
keep  it  in  a  cool  dark  place  and  to  shake  it  thoroughly  in  order 
to  wash  down  its  inner  wall  before  using.  It  is  also  advisable  not 
to  make  up  too  large  a  quantity  of  solution  of  a  substance  that 
acts  upon  the  glass.  Any  solution  about  whose  exact  strength 
there  is  any  doubt  should  be  restandardized. 


112  QUANTITATIVE  ANALYSIS 


Acidimetry  and  Alkalimetry 

The  processes  of  Acidimetry  and  Alkalimetry  are  those  by  which 
the  strength  of  acids  and  bases  are  determined. 


STEPS  TO  BE  CONSIDERED  IN  PREPARING  AND  STANDARDIZING 

HALF-NORMAL  SOLUTIONS  OF  HYDROCHLORIC  ACID  AND 

SODIUM  HYDROXID 

(1)  Preparation  of  an  approximately  half -normal  solution  of 
hydrochloric  acid. 

(2)  Preparation  of  an  approximately  half-normal  solution  of 
sodium  hydroxid. 

(3)  The  titration  of  the  acid  against  the  alkali  for  the  deter- 
mination of  the  relative  values  of  the  approximately  half-normal 
solutions  of  hydrochloric  acid  and  sodium  hydroxid. 

(4)  The  use,  through  factors,  of  approximately  half-normal  solu- 
tions as  standard  solutions. 

(5)  The  determination  with  Iceland  spar  or  pure  sodium  car- 
bonate of  the  absolute  value  of  the  approximately  half-normal 
solution  of  hydrochloric  acid  and  the  preparation  of  an  exactly  half- 
normal  solution  of  hydrochloric  acid. 

(6)  The  preparation  of  an  exactly  half-normal  solution  of  sodium 
hydroxid  by  use  of  the  previously  established  values. 

(7)  The  determination  with  oxalic  acid  of  the  absolute  value  of 
the  approximately  half-normal  solution  of  sodium  hydroxid  and 
the  preparation  of  an  exactly  half-normal  solution  of  sodium 
hydroxid. 

(8)  The  reestablishment  of  the  relative  values  of  the  two  solu- 
tions, which,  if  correctly  diluted,  should  now  be  equal,  volume  for 
volume. 


VOLUMETRIC  ANALYSIS  113 


(1)  Preparation  of  an  Approximately  Half -Normal  Solution  of 
Hydrochloric  Acid 

Theoretically  the  simplest  way  to  prepare  accurate  standard 
solutions  is  to  dissolve  calculated  and  carefully  measured  amounts 
of  substances  in  an  exact  amount  of  solvent.  As  a  matter  of  fact, 
however,  neither  the  conditions  nor  the  chemicals  used  in  most 
cases  are  sufficiently  to  be  relied  upon  to  give  perfect  results. 

An  approximately  normal  solution  can  be  made  and  its  relative 
value  to  an  exactly  normal  solution  can  be  determined  or  an  exactly 
normal  solution  can  be  prepared  from  the  one  that  is  approximately 
normal. 


CALCULATION  OF  THE  NUMBER  OF  CUBIC  CENTIMETERS  OF 

AQUEOUS  HYDROCHLORIC  ACID  REQUIRED  TO  MAKE  A 

LITER  OF  A  HALF-NORMAL  SOLUTION 

As  a  solution  of  hydrochloric  acid  gas  is  to  be  used,  it  is  necessary 
to  calculate  the  weight  of  the  hydrochloric  acid  gas  contained  in 
the  aqueous  hydrochloric  acid. 

A  normal  solution  of  HC1  should  contain  36.458  grams  of  HC1 
gas  in  one  liter. 

At  15°  C.,  HC1  (sp.  gr.  1.2)  contains  39.1%  of  HC1  by  weight. 
In  100  c.c.  of  aqueous  HC1  (sp.  gr.  1.2)  there  are  46.92  grams  of 
HC1  gas  for: 

100  c.c. 

1.2 

120  grams  =  weight  of  100  c.c.  of  aqueous  HCL 

.391  X  120  grams  =  46.920  grams. 
46.92  grams  =  weight  of  HC1  gas  in  100  c.c.  of  aqueous  HCL 


114  QUANTITATIVE  ANALYSIS 

To  furnish  36.458  grams  of  HC1  gas,  there  must  be  77.70  c.c.  of 
aqueous  hydrochloric  acid  (sp.  gr.  1.2)  according  to  the  following 
proportion: 

.       46.92  :  36.458  ::  100  :  x 
x  =  77.70  c.c. 

N 
In  a  liter  of  a  normal  solution  of  hydrochloric  acid,  -y  >   then, 

there  are  77.7  c.c.  of  aqueous  hydrochloric  acid  (sp.  gr.  1.2). 

N 
In  a  half-normal  solution,  -^,  there  are  in  every  liter  one-half  of 

77.7  c.c.  or  38.85  c.c.  of  aqueous  HC1  (sp.  gr.  1.2)  measured  at 
15°  C. 


PROCEDURE 


In  a  graduated  cylinder  measure  a  volume  approximately  equal 
to  38.8  c.c.  of  aqueous  hydrochloric  acid  (sp.  gr.  1.2)  and 
about  one-tenth  as  much  in  addition.86 

Add  enough  distilled  water  to  dilute  to  one  liter  and  shake87  for 
a  minute  or  more.     If  the  cylinder  is  too  full 
to  allow  the  liquid  to  be  shaken  thoroughly,   Preparation  of  the 

u      i  j.-  J    standard  acid  solu- 

transfer  to  a  beaker,  stir  vigorously  and   tion 
return  the  solution  to  the  bottle. 

N 
Label  the  bottle  "  Approximately  --HCl". 


EXPLANATORY  FACTS 

86.  The  extra  volume  is  added  to  make  sure  that  it  is  stronger 
than  half-normal.     It  is  easier  later  to  dilute  than  to  add  an 
exact  amount  of  acid. 

87.  It  is  of  the  utmost  importance  to  get  the  solution  into  a 
homogeneous  state. 


VOLUMETRIC   ANALYSIS  115 


?)  Preparation  of  an  Approximately  Half-Normal  Solution  of 
Sodium  Hydroxid® 

PROCEDURE 

Calculate  in  the  notebook  the  amount  of  caustic  soda,  NaOH, 
necessary  to  make  one  liter  of  a  half-normal  solution. 

Weigh89  out  on  a  rough  balance  in  a  counterbalanced  porcelain 
dish  an  amount  equal  to  the  calculated  weight  plus  one- 
tenth  as  much  in  addition.90 

Measure  1000  c.c.  of  distilled  water  in  a  measuring  cylinder. 

Dissolve  the  sodium  hydroxid  in  a  part  of  this  Preparation  of  the 
distilled  water  contained  in  a  beaker  or  standard  aikaii 
porcelain  dish. 

Stir  until  the  hydroxid  is  all  dissolved  and  then  cover  with  a 
watch  glass  till  it  is  somewhat  cooled. 

Transfer  the  solution  to  a  bottle,  add  the  rest  of  the  distilled 
water  and  shake  vigorously. 

Label  the  bottle,  which  must  have  a  rubber  stopper  or  a 
glass  stopper  light ly  covered  with  vaseline,  — 

"Approximately     NaOH  ". 


EXPLANATORY  FACTS 

88.  Standard  acid  solutions  may  be  kept  for  some  time  with- 
out deterioration.     This  is  not  the  case  with  standard  alkali 
solutions  which  absorb  carbon  dioxid  from  the  air  and,  as  has 
been  said,  act  upon  the  glass  of  the  bottles  in  which  they  are  con- 
tained.    It  is  therefore  important  to  titrate  the  alkali  frequently 
with  a  standard  acid  in  order  to  reestablish  its  strength. 

89.  Weigh  quickly  and  do  not  leave  exposed  to  the  air,  as 
caustic  soda  is  hygroscopic.     Cork  the  stock  bottle  at  once. 

90.  Sodium  hydroxid,  caustic  soda,  is  an  excellent  alkali  for 
general  use;  it  is  a  strong  base,  forms  soluble  salts  with  all  acids 
and,  being  readily  soluble  (133.3  parts  in  100  parts  of  water),  may 
be  made  up  to  a  strong  solution. 


116  QUANTITATIVE  ANALYSIS 


(3)   The  Titration  of  the  Acid  against  the  Alkali 

and 

The  Calculation  of  the  Relative  Values  of  the  Approximately 

Half-Normal  Solutions  of  Hydrochloric 

Acid  and  Sodium  Hydroxid 


PROCEDURE 

Wash  two  burets  thoroughly  with  distilled  water.     When  they 

are  emptied,  no  drops  should  adhere  to  the  inner  walls. 

If  they  do,  the  surface  is  not  clean  and  should  be  treated 

with  "chromic  acid  mixture." 
Put  a  label  on  the  top  front  of  each  buret,  one  Labeling  the 

marked  "HC1"  and  the  other  "NaOH".   b«*ets 
Fit  a  cork  to  each  and  label  each  cork,  one  marked  "HC1"  the 

other  "NaOH".     Keep  a  small,  properly  labeled  watch 

glass  upon  which  to  lay  each  cork. 
Close   the   outlet   of   the   "HC1"   buret   and 

.    x.  .  Filling  the  burets 

pour  in  5  c.c.  of  the  again  well-shaken 

IHCI. 

Hold  the  stoppered  buret  in  a  nearly  horizontal  position  and 
allow  the  acid  to  flow  over  the  entire  interior  and  shake. 
Then  remove  the  cork  and  let  the  acid  run  out  through 
the  outlet. 

This  should  be  repeated. 

Treat  the  "NaOH"  buret  in  the  same  way.  (If  it  is  certain 
that  the  burets  are  absolutely  dry,  the  foregoing  may  be 
omitted.) 

Put  a  small  glass  funnel  into  the  top  of  each  buret  with  the 
stem  against  the  inner  wall  and  fill  each  buret  with  its  well- 
shaken  solution. 


VOLUMETRIC  ANALYSIS  117 

Let  enough  of  each  liquid  run  through  the  tips  to  insure  the 

removal  of  any  air  bubbles. 
If  there  are  any  drops  of  the  liquid  left  on  the  inner  walls  of  the 

burets  above  the  zero  mark,  absorb  with  a  filter  paper  rolled 

around  a  glass  rod. 

Be  sure  that  the  burets  are  perfectly  vertical. 
Run  out  the  liquid  in  each  buret  till  the  bottom  of  the  meniscus 

stands  at  the  zero  mark.     If  a  "blue  line"  buret  is  used, 

bring  the  constriction  seen  on  the  blue  line  to  the  zero  mark. 
If  the  liquid  is  below  the  zero  mark,  make  a  careful  reading  at 

such  a  point. 
Record  the  reading  in  the  notebook.       It  is  well  to  keep  the 

HC1  and  NaOH  readings  in  the  same  relative  positions  on 

the  page  as  the  positions  of  the  burets  (see  next  page). 
Run  about  40  c.c.  of  the  add  into  a  casserole,  an  Erlenmeyer  or 

an  ordinary  flask,  containing  about  40  c.c.  of  distilled  water, 

and   add   two   or   three   drops   of  methyl 

.     ..   _,  m  •  -xi  Titration  process 

orange  indicator.     To  mix  well,  stir  with  a 
rod  or  shake  the  flask  with  a  rotary  motion. 

Include  any  drop  hanging  from  the  tip  of  the  buret  by  touching 
it  with  the  rod  or  with  the  inside  of  the  neck  of  the  flask 
and  washing  it  down  with  water  from  a  wash  bottle. 

Run  into  the  container  about  5  c.c.  less  of  the  sodium  hydroxid 
than  that  used  of  the  hydrochloric  acid  and  then  slowly  con- 
tinue to  add  the  sodium  hydroxid  solution,  drop  by  drop,  till 
the  pinkish  red  solution  turns  yellow.  Again  do  not  neglect 
any  hanging  drops  of  the  solution.  Wash  down  the  sides  of 
the  dish  with  distilled  water. 

Place  the  container  under  the  HC1  buret  and  continue  to  change 
from  one  buret  to  the  other  till  one  drop  of  the  HC1  or  one 
drop  of  the  NaOH  changes  the  color. 

As  the  pink  is  the  sharper  of  the  two  color  reactions,  finish  by 
adding  a  drop  of  HC1. 

After  a  few  minutes'  wait  for  it  to  drain,  note  the  reading  of  the 
HC1  buret. 


118 


QUANTITATIVE  ANALYSIS 


Continuing  this,  make  six  successive  readings  of  these  repeated 
changes  and  record  as  follows: 


Readings. 
HC1 
level  at  start,  1.34 

Actual  number  of 
c.c.  used. 

41.83 
1.34 
40.49 

Relative  Values. 

Actual  number  of 
c.c.  used. 
39.1 
0.5 
38.6 

Readings. 
NaOH 

levelatstart,0.5 

c.c. 
41.83 
41.98 
42.15 
42.29 
42.49 
42.69 

c.c. 

40.49 
40.64 
40.81 
40.95 
41.15 
41.35 

.048 
.048 
.048 
.047 
.047 
.047 

c.c. 

38.6 
38.75 
38.93 
39.08 
39.27 
39.45 

C.C. 

39.1 
39.25 
39.43 
39.58 
39.77 
39.95 

Average  1.0475 

Therefore  1  c.c.  of  NaOH  solution  is  equivalent  to  1.0475  c.c. 
of  HC1  solution. 


Shake  the  bottles  containing  the  half-normal  solutions,  refill  the 

burets  and  repeat  the  titration. 
These  results  should  check  within  0.2  per  cent,  of  the  whole  ratio. 


VOLUMETRIC  ANALYSIS  119 


(4)   The  Use  —  Through  Factors  —  of  Approximately  Half-Normal 
Solutions  as  Standard  Solutions. 


Those  limited  in  time  need  not  prepare  the  exactly  half-normal 
solutions,  since  the  value  of,  or  the  degree  of  normality  of,  the 
approximately  half-normal  solutions  may  be  determined  by 
calculation. 


It  is,  however,  oftentimes  desirable  to  have  exact  solutions. 
Thus,  as  shown  in  the  accompanying  sample  calculation  (see 

page  126),  38.24  c.c.  of  exactly  ^HCl  dissolves  0.9567  gram  of 

Zi 

CaC03  whereas  this  same  weight  of  CaC03  dissolved  ha  34.49  c.c. 
of  this  particular  solution  of  HC1. 

N  38  24 

The  approximately  -^  solution  of  HC1  is  therefore  ^-p  ^  =  1,108 


tunes  stronger  than  semi-normal,  the  factor  of  normality  for  the 

N 
approximately  77-  HCl  solution. 


N 
A  given  number  of  cubic  centimeters  of  an  exact  -~  NaOH 

solution  is  exactly  equivalent  to  the  same  number  of  cubic  centi- 

N 

meters  of  an  exact  -5-  HCl  solution.     But  in  establishing  the  abso- 
lute value  of  the  HCl  solution  (see  sample  calculation  p.  125)  it 

N 
was  found  that  40.0  c.c.  approximately -^-NaOH  solution  neutra- 

N 
lized  43.65  c.c.  of  approximately  -_•  HCl  solution. 

By  using  the  factor  of  normality  of  this  HCl  solution,  found  above 


120  QUANTITATIVE  ANALYSIS 


N 
to  be  1.108,  it  is  shown  that  43.65  c.c.  of  this  approximately  -^  HC1 

is  equivalent  to  48.36  c.c.  of  exact  —  HC1  solution. 

AQ     C)f* 

'      =  1.209,  the  factor  of  normality  for  the  approximately 
~  NaOH  solution. 


The  amount  of  approximately  half-normal  solution  used  can 
always  be  expressed  in  terms  of  the  exactly  half-normal  solution 
by  multiplying  the  number  of  cubic  centimeters  used  by  the  factor 
of  its  normality. 

If  a  solution  is  not  exactly  normal,  the  factor  of  its  normality 
should  be  written  on  the  label. 


VOLUMETRIC  ANALYSIS  121 


(5)  The  Determination  with  Iceland  Spar*  of  the  Absolute  Value  of 
the  Approximately  Half -Normal  Solution  of  Hydrochloric  Acid 

and 

The  Preparation  of  an  Exactly  Half-Normal  Solution  of 
Hydrochloric  Add 


(a)  The  solution  of  the  Iceland  spar,  CaCOs,  in  an  excess  of 
the  approximately  half-normal  solution  of  hydrochloric 
acid. 

(6)  The  titrating  back  of  the  excess  of  acid  with  the  approxi- 
mately half-normal  solution  of  sodium  hydroxid. 

(c)  The  addition  of  the  calculated  amount  of  distilled  water 
to  the  approximately  half-normal  solution  of  hydro- 
chloric acid. 


REACTIONS 

(I)  CaC03  +  2  HC1  =  CaCl2  +  H2O+  CO2. 
(II)  HC1  4-  NaOH  =  NaCl  +  H2O. 


*  If  their  purity  and  value  is  guaranteed,  sodium  carbonate  or  precipitated 
calcium  carbonate  may  be  used  instead  of  Iceland  spar. 


122  QUANTITATIVE  ANALYSIS 


THE  DETERMINATION  OF  THE  ABSOLUTE  VALUE 

Pure  calcium  carbonate  (Iceland  spar)  requires,  of  course,  an 
exact  amount  of  hydrochloric  acid  to  dissolve  it.  The  acid  can 
therefore  be  standardized  against  it. 

PROCEDURE 

Weigh  into   Erlenmeyer  or  ordinary  flasks 9l   weighing  and  dis- 
two  portions  of  about  0.5  of  a  gram  each   solvins the  i«*iand 

spar  in  a  definite 

of  Iceland  spar  that  has  been  ground  to   volume  of  hydro- 
an  impalpable  powder.92  chloric  acid 

N 
Add  about  30  c.c.  of  the  -=  HC1  from  the  buret,  make  a  note  of 

the  exact  amount  added. 

Let  this  stand  till  the  Iceland  spar  is  wholly  dissolved.  Hold  to 
the  light  to  see  if  there  are  any  particles  undissolved. 
(While  the  Iceland  spar  is  dissolving,  begin  to  grind  the  iron 
ore  according  to  the  directions  under  "The  Volumetric 
Determination  of  Iron  in  an  Ore,"  page  150.) 


EXPLANATORY  FACTS 

91.  If  dissolved  in  a  flask,  loss  by  effervescence  is  avoided. 
If,  however,  it  is  necessary  to  use  a  beaker,  keep  it  covered.     As 
the  CaCO3  is  somewhat  slow  to  dissolve,  fall-strength  acid  solu- 
tion acts  as  a  better  solvent. 

92.  Gritty  particles  dissolve  slowly  and  with  difficulty.    Failure 
to  dissolve  completely  the  Iceland  spar  will  result  in  error  and  will 
ruin  subsequent  analyses. 


VOLUMETRIC   ANALYSIS  123 

Add  two  or  three  drops  of  the  methyl  orange  indicator  and  wash 
down  the  inside  of  the  flask  with  water  from  a  wash  bottle. 

Make  the  initial  reading  on  the  NaOH  buret  and  run  in  the 
NaOH  till  the  indicator  shows  that  the  so-  Titrating  back  the 
lution  has  just  become  alkaline.  excess  of  acid 

Again  note  the  NaOH  reading. 

In  case  there  is  any  doubt  as  to  whether  the  exact  end  point  was 
reached,  titrate  back  with  the  hydrochloric  acid  and  proceed 
to  get  an  exact  end  point  with  the  sodium  hydroxid.93 

Make  careful  readings. 

Calculate  the  excess  of  acid  which  was  used  and  then  the  amount 
of  acid  which  combined  with  the  calcium  carbonate.  This 

N 
gives  the  amount   of  approximately  -~  HC1  necessary  for 

this  reaction.     Calculate  the  theoretical  amount  of  exactly 

N 

-=•  HC1  necessary  for  this  reaction.     A  comparison  of  these 

Zi 

N 
figures  shows  how  much  the  approximately  -~  HC1  needs 

N 
diluting  to  make  it  exactly  -5- . 

N 
If  the  student's  time  admits  of  the  standardization  of  the    -^ 

solution  of  NaOH  against  oxalic  acid,  this  standardization  must 

N 
be  done  before  the  approximately  -^  solution  of  HC1  is  diluted  to 

N 
exactly  -~  and  while  the  relative  values  of  the  acid  and  alkali 


EXPLANATORY  FACT 

93. '  A  certain  number  of  the  cubic  centimeters  of  the  acid  have 
been  used  in  reacting  with  the  calcium  carbonate.  The  excess 
of  acid  above  this  amount  has  now  been  determined  by  titrating 

back  with  -~-  NaOH  solution. 


124  QUANTITATIVE  ANALYSIS 

solutions  are  still  known.     In  which  case,  the  acid  may  be  diluted  to 

N  N 

exactly  77  after  the  standardization  of  the  approximately  -^  NaOH 

&  & 

against  oxalic  acid.     Otherwise,  proceed  with  the  dilution  at  this 
point  (Calculation,  page  127). 


THE  PREPARATION  OF  THE  EXACTLY  HALF-NORMAL  SOLUTION 


Measure  accurately  the  amount  of  water  to  be  added  to  the 
number  of  cubic  centimeters  of  standard  Dilution  of  fh9  ap_ 
solution  measured.  proximate*  5  HCI 

Add  this  exact  amount  from  a  graduated  flask,   solutiontomiean 
pipet  or  buret.  exactly  ?solution 

Shake  the  solution  till  thoroughly  homogeneous. 


VOLUMETRIC   ANALYSIS  125 


SAMPLE   CALCULATIONS 

FOR  THE  DETERMINATION  OF  THE  ABSOLUTE  VALUE 

AND  OF 

N 
THE  QUANTITIES  TO  MAKE  AN  EXACTLY  -=•  SOLUTION  OF  HC1 


N 
If  it  takes  40  c.c.  of  an  approximately  -^  NaOH  solution  to 

N 
neutralize  43.65  c.c.  of  an  approximately  -^  HC1  solution,  then  1  c.c. 

A 


of  ^  NaOH  =  ^por  1.091  c.c.  of  ^  HC1  ("Relative  Value"  of 
the  two  solutions). 

Amount  of  -^  HC1  added  to  the  Iceland  Spar  =  45.40  c.c. 

N 
Amount  of  -^  NaOH  used  to  titrate  excess  of  acid  =  10.00  c.c. 


N 
10.00  c.c.  of  this  approximate  -~  NaOH  =  10.91  c.c.  in  terms  of 

N 
this  approximate  -~  HC1. 


10.91  c.c.  of  HC1  was  neutralized  by  NaOH. 
45.40  c.c. 
10.91  c.c. 


Therefore  34.49  c.c.  were  used  to  dissolve  the  Iceland  Spar. 


126  QUANTITATIVE  ANALYSIS 


ABSOLUTE  VALUE 

0.9567  g.  CaCO3  required  for  solution  34.49  c.c.  ^  HC1. 

& 

CaC03  +  2  HC1  =  CaCl2  +  H2O  +  CO2. 
100.07  (CaC03)  :  72.9  (2  HC1)  =  0.9567  :  x 

x  =  0.6969  gram  of  HC1. 

That  is,  34.49  c.c.  contain  0.6969  gram  of  HC1  and  therefore 
1  c.c.  contains  0.0202  gram. 


QUANTITIES  TO  MAKE  AN  EXACTLY  —  SOLUTION 

N 
34.49  c.c.  of  the  approximately  ^-HCl   solution  are  needed  to 

dissolve  0.9567  gram  of  CaC03. 

How  many  cubic  centimeters  of  an  exact  -~  solution  would  it 
take? 

The  equation  in  the  foregoing  section  shows  that  it  takes  two 
molecules  of  HC1  to  dissolve  one  molecule  of  CaCO3. 

Molecular  weight  of  CaCO3  =  100.07. 

One-half  the  molecular  weight  in  grams  =  50.035. 

With  a  normal  solution  of  HC1,  a  liter  (1000  c.c.)  is  equivalent  to 
50.035  grams  of  calcium  carbonate,  therefore  a  liter  of  half-normal 
HC1  is  equivalent  to  25.017  grams. 

25.017  :  1000  ::  0.9567  :  x 
25.017  x  =  956.70 
x  =  38.24 

N 
It  takes,  then  38.24  c.c.  of  an  exactly  -=-  HC1  solution  to  dissolve 

0.9567  gram  of  calcium  carbonate. 


VOLUMETRIC  ANALYSIS  127 


Since  it  took  less  of  the  approximately  -^  HC1  solution  than  it 
would  have  taken  were  the  solution  exactly  half  normal,  it  is  too 
strong. 

38.24 

34.49 

3.75 


Every  34.49  c.c.  of  the  approximately  ^-  HC1  should  be  diluted 
with  3.75  c.c.  of  distilled  water  to  make  it  exactly  semi-normal. 
If  there  are  500  c.c.  of  the  approximately  ^  HC1,  ^^  X  3.75  = 
54.3  c.c.  which  is  the  number  of  cubic  centimeters  of  water  to  be 
added  to  make  the  HC1  solution  exactly  half-normal. 


Fractional  volumes  should  be  measured  with  a  buret.  Measure- 
ments may  be  made  by  using  a  measuring  flask  nearest  in  size  to 
the  volume  required  and  the  rest  of  the  volume  added  from  a 
buret.  By  filling  them  as  many  times  as  necessary,  burets  only 
may  be  used. 


128  QUANTITATIVE  ANALYSIS 


(6)  The  Preparation  of  Exactly  Half-Normal  Solution  of  NaOH 
from  Previously  Established  Values 

If  time  is  limited,  the  amount  of  water  required  Diluti(m  of  o^  ap_ 

to  be  added  to  the  approximately  half-normal  ^,0,^^  ?  NaOH 

solution  of  NaOH  to  make  it  exactly  half-normal  solutioll  to  ,^ke  „, 

can  be  calculated  from  the  data  already  obtained  exactly  N  8olution 
as  follows: 

1  c.c.  ^  NaOH  was  found  to  equal  1.091  c.c.^HCl. 


If  34.49  c.c.  of  approximately  -^  HC1  are  diluted  with  3.75  c.c. 
of  distilled  water  to  make  an  exactly  -~  solution  of  HC1.  then 
f^f  =  31.61  and  ^=3.43,0. 

Therefore,  in  this  case,  every  31.61  c.c.  of  approximately  -^  NaOH 
should  be  diluted  with  3.43  c.c.  of  distilled  water  to  make  an  exactly 
2~  solution  of  NaOH,  or,  to  each  cubic  centimeter  of  the  remaining 
solution,  add  0.108  of  a  cubic  centimeter  of  water. 


PROCEDURE 

Dilute  the  alkali  as  described  under  the  directions  for  dilution 
of  the  acid. 


VOLUMETRIC   ANALYSIS  129 


(7)   The  Determination  of  the  Absolute  Value  of  the  Approx- 
imately Half-Normal  Solution  af  Sodium  Hydroxid 
with  Oxalic  Acid 

and 

The  Preparation  of  an  exactly  Half-Normal  Solution  of 
Sodium  Hydroxid 


(a)  The  titration  of  a  solution  of  oxalic  acid  with  the  ap- 
proximately half-normal  solution  of  sodium  hydroxid. 

(6)  Titration  back,  with  the  approximately  half-normal  solu- 
tion of  hydrochloric  acid,  of  the  excess  of  alkali  added. 

(c)  Calculation  and  addition  of  the  necessary  amount  of  dis- 
tilled water  to  the  approximately  half-normal  solution 
of  sodium  hydroxid. 


REACTIONS 

(I)  2  NaOH  +  H2C204  =  Na2C2O4  +  2  H2O. 
(II)  NaOH  +  HC1  =  NaCl  +  H2O. 


130  QUANTITATIVE  ANALYSIS 


THE  DETERMINATION  OF  THE  ABSOLUTE  VALUE 
PROCEDURE 

Weigh  into  two  casseroles  two  portions  of  pure,  crystallized 

oxalic  acid9*  (H2C2O4.2  H2O)  of  about  0.8  gram  each. 
Dissolve  in  about  50  c.c.  of  water. 
Heat  to  boiling  and  add  a  few  drops  of  phenolphthalein  indicator. 

N 
Titrate  hot  with  —  NaOH  solution  till  it  pro-  Titration  of  the 

duces  a  pink  color  which  lasts  for  a  few 

minutes. 

N 
Add  from  a  half  to  one  cubic  centimeter  of  the  -~  HC1  solution 

noting  the  readings. 
Bring  to  a  boil. 


EXPLANATORY  FACT 

94.  If  oxalic  acid  of  this  exact  composition  is  not  available,  it 
may  be  prepared  by  Winkler's  method  ("  Uebungen  in  der  Mass- 
analyse,"  p.  69) .  500  grams  of  oxalic  acid  are  dissolved  in  500  grams 
of  boiling  HC1  (1.07  sp.  gr.).  It  is  then  allowed  to  crystallize  by 
stirring  the  solution  in  a  dish  placed  in  ice  water.  The  crystals 
are  filtered  through  glass  wool  and  washed  with  HC1.  The  yield 
is  then  redissolved  in  boiling  hydrochloric  acid,  crystallized  and 
filtered  as  before.  This  time,  however,  the  crystals  are  washed  with 
a  little  water  and  redissolved  in  just  enough  boiling  water.  The 
crystals  obtained  by  cooling  this  solution  are  filtered  and  washed 
with  water  and  recrystallized  at  least  twice.  The  final  crystals  are 
then  dried  over  a  desiccating  agent  which  must  be  frequently 
changed.  It  is  now  free  from  chlorin  and  mineral  matter. 


VOLUMETRIC  ANALYSIS  131 

N 
Titrate  back  with  the  -^  NaOH  solution. 

Calculate  the  excess  of  alkali  added  and  then  the  amount  of 


alkali  which  combined  with  the  oxalic  acid.     This  gives  the 
amount  of  the  a 
for  this  reaction. 


N 
amount  of  the  approximately  -   NaOH  solution  necessary 


THE  PREPARATION  OF  AN  EXACTLY  HALF-NORMAL  SOLUTION 
OF  SODIUM  HYDROXID 

PROCEDURE 


N 
Calculate  the  amount  of  exactly  -~  NaOH  solution  necessary  for 

this  reaction.     A  comparison  of  these  figures  shows  how 

N 
much  the  approximately  -77  NaOH  needs  diluting  to  make 

£i 

it  exactly  -_•• 


132  QUANTITATIVE  ANALYSIS 


SAMPLE   CALCULATION 
FOR  THE  DETERMINATION  OF  THE  ABSOLUTE  VALUE 

AND   THE 

DETERMINATION  OF  THE  QUANTITIES  TO  BE  ADDED  TO  MAKE 

N 
AN  EXACTLY  —  SOLUTION  OF  NaOH 

When  standardized  with  oxalic  acid, 


H2C2O4.2  H2O  +  2NaOH  =  Na2C2O4  +  4  H20 
126.05  80.02 

Weight  of  oxalic  acid  used  =  0.7500  gram. 

N 
Volume  required  of  approximately  -^  NaOH  used  in  this  titra- 

tion,  20  c.c. 

126.05  :  80.02  =  0.7500  :  x 

x  =  0.4761  gram  NaOH. 

Therefore  0.7500  gram  H2C2O4.2  H2O  requires  0.4761  gram  NaOH. 
Therefore  20  c.c.  of  this  solution  contain  0.4761  gram  NaOH, 
and  1  c.c.  of  this  solution  contains  0.0238  gram  NaOH  or  this 
solution  contains  23.8  grams  NaOH  per  liter. 

If  the  solution  were  exactly  half-normal,  it  would  contain  20.004 
grams  NaOH  per  liter  or  .020  gram  in  1  c.c. 

It  is  therefore  too  strong  and  should  have  added  to  it  76.9  c.c. 
of  water  according  to  the  following: 

Should  there  remain  405  c.c.  of  the  solution,  let  x  equal  the 

N 
volume  required  to  make  the  solution  exactly  -^  • 

Then  405  X  0.238  =  x  X  0.020. 

x  =  481.9  c.c. 
481.  9  c.c.  —  405  c.c.  =  76.9  c.c.,  the  volume  to  be  added. 


VOLUMETRIC  ANALYSIS  133 


(8)   The  Reestablishment  of  the  Relative  Values  of  the  two 
Solutions 


If  time  permits,  the  correctness  of  the  preceding  work  may  be 
verified  by  the  reestablishment  of  the  relative  values  of  the  two 
adjusted  solutions,  equal  volumes  of  which  should  now  neutralize 
each  other. 


PROCEDURE 

Take  an  accurately  measured  volume  of  from  10  to  20  c.c.  of 

either  solution. 

Use  methyl  orange  as  an  indicator. 
Determine  the  volume  of  the  other  solution  necessary  to  effect 

neutralization. 


134  QUANTITATIVE  ANALYSIS 


THE  VOLUMETRIC  DETERMINATION 

OF  THE 
TOTAL  ALKALI  IN  SODA  ASH95 

A  Typical  Saturation  Process 

This  method  is  applicable  to  the  titration  of  caustic  soda,  caustic 
potash,  or  alkali  carbonates. 

Soda  ash,  NasCOs,  etc.,  is: 

(a)  dried  to  determine  the  moisture; 

(6)  neutralized  with  standard  hydrochloric  acid; 

(c)  calculated  as  "  total  available  alkali." 

TYPE  REACTIONS 

(I)  NasCOs  +  2  HC1  =  2  NaCl  +  H20  +  C02. 
(II)  NaOH  +  HC1  =  NaCl  +  H2O. 


EXPLANATORY  FACT 

95.  Soda  ash  is  calcined  crude  sodium  carbonate  resulting  from 
either  the  "Le  Blanc"  or  the  "Ammonia"  process  for  making 
soda.  It  contains  a  little  sodium  hydroxid,  often  traces  of  sulfids, 
thiosulfates,  sulfites,  sulfates,  silicates  and  chlorids.  There  also 
may  be  present  alumina  and  ferric  oxid.  "Le  Blanc"  soda  has, 
as  its  chief  impurity,  sodium  sulfate,  while  "Ammonia"  soda  is 
generally  free  from  caustic  soda,  (NaOH),  sulfid  and  sulfate,  and 
contains  the  most  chlorid.  "  Ammonia  "  soda  (sp.  g.  0.8)  is  lighter 
than  "  Le  Blanc  "  soda  (sp.  g.  1.2). 


VOLUMETRIC  ANALYSIS  135 


PROCEDURE  96 

Weigh  an  ignited  platinum  crucible. 

Weigh  about  5  grams 97  of  the  sample  into  the  crucible. 

Heat  to  dull  redness  for  about  twenty  minutes.98  Expelling  the  mois- 

Cool  in  a  desiccator.  ture  from  the  soda 

Weigh  and  repeat  the  heating  for  a  few  minutes   ash 

until  a  constant  weight  is  reached. 
Calculate  the  loss  in  weight  as  moisture. 
Transfer  the  contents  of  the  crucible  to  a  500  c.c.  (No.  4)  beaker. 
Wash  the  crucible  thoroughly  and  catch  the  preparation  Of  the 

washings  in  the  beaker.  solution  of  the  soda 

Add  about  100  c.c.  of  water  and  stir. 
Warm,  if  necessary,  till  all  is  dissolved  that  will  dissolve. 
Transfer  this  solution  quantitatively  through  a  filter  directly 

into  a  250  c.c.  volumetric  flask.  patration  of  the 

Wash   the   filter  paper  and   insoluble  residue   solution 

with  a  stream  of  hot  water  from  a  water  bottle  till  a  drop 

of  the  filtrate  gives  no  further  reaction  with  litmus  "paper. 


EXPLANATORY  FACTS 

96.  In  the  analysis  of  soda  ash,  German  alkali  manufacturers 
(1)  always  ignite  the  soda  ash  before  determining  the  per  cent,  of 
alkali  and  calculate  the  results  on  the  ignited  material  and  (2) 
they  include  in  the  total  per  cent,  of  alkalinity,  that  which  is  due  to 
the  insoluble  portion,  calcium  and  magnesium  carbonates,  ferric 
oxid,  etc.,  as  well  as  the  soluble  portion.     Since  the  total  insoluble 
matter  (including  nonalkaline  substances  like  sand)  is  very  small, 
there  is  but  slight  difference  in  the  percent,  of  alkalinity  in  the  soda 
ash  whether  analyzed  by  the  German  or  other  methods. 

97.  In  the  above  determination,  an  unignited  sample  is  weighed 
and  the  soluble  alkali  only  is  determined. 

98.  If  this  temperature  is  exceeded,  the  mass  will  fuse. 


136  QUANTITATIVE  ANALYSIS 

Cool  to  the  "graduation  temperature"99  of  the  flask  and  add 

water  up  to  the  graduation  line.     Some  of  this  water  that 

is  added  should  be  directed  down  all  parts  of  the  inside  of 

the  neck  of  the  flask  to  make  sure  that  all  of  the  solution 

is  collected  in  its  bulb. 
Dry  the  inner  neck  down  to  the  liquid  level,  using  a  filter  paper 

rolled  around  a  glass  rod. 
Mix  the  liquid  thoroughly  by  pouring  it  back  and  forth  from 

the  flask  into  a  dry  beaker.100  Preparation  of 

After  this  keep  the  flask  stoppered.  <di«uot  P*** 

Titrations  are  to  be  made  with  aliquot  parts  of  50  c.c.  each  of 

this  solution.    This  amount  may  be  measured  in  pipets  or 

volumetric  flasks  of  this  capacity. 
Rinse  twice  the  selected  measuring  instrument  with  a  little  of 

the  solution  before  the  aliquot  part  is  measured.101 
Pour  the  measured  50  c.c.  into  a  clean  250  c.c.  flask. 
Wash  out  the  graduated  flask  quantitatively  into  the  titrating 

flask. 
Dilute  the  50  c.c.  to  about  100  c.c. 


EXPLANATORY  FACTS 

99.  The  graduation  temperature  is  usually  marked  on  gradu- 
ated measuring  apparatus.     It  is  about  room  temperature,  that  is, 
60°  F.,  or  17.5°  C.,  a  temperature  decided  upon  by  Mohr.     A 
true  liter  is  the  volume  of  one  thousand  grams  of  water  at  4°  C. 
As  a  more  practical  unit  one  thousand  grams  of  water  at  17.5°  C. 
is  taken  and  is  called  the  "Mohr  Liter." 

100.  Since  the  solution  has  now  been  mixed  sufficiently  to 
make  it  homogeneous  and  since  only  aliquot  parts  of  it  are  to  be 
used  for  titration  neither  that  adhering  to  the  beaker  nor  that 
which  is  to  be  used  for  rinsing  the  pipet  or  50  c.c.  flask  need  be 
considered. 

101.  Of  course  the  amount  of  soda  ash  in  each  aliquot  part  cor- 
responds to  one-fifth  of  the  original  weight  taken. 


VOLUMETRIC  ANALYSIS  137 

Add  three  or  four  drops  of  methyl  orange  solution.102 

From  the  buret,  after  noting  the  reading,  add  a  slight  excess  of 

£  HO. 

N 
Note  the  reading  and   titrate  back   with  -^  NaOH   and  find 

the  point  at  which  a  drop  of  either  solution   xitration  of  the 

changes  the  color  from  yellow  to  pinkish   aasa^ae  solution 

red  or  vice  versa. 
End  the  titration  with  the  acid. 
Take  readings  for  both  acid  and  alkali. 
Repeat  this  determination  with  another  50  c.c.  of  the  soda  ash 

solution. 


EXPLANATORY  FACT 

102.  Methyl  orange  is  unaffected  by  the  CO2  that  is  liberated 
when  the  acid  is  added  to  the  carbonate.  Most  indicators  show 
an  acid  reaction  with  the  C0%  which  makes  the  alkali  solution 
appear  to  be  neutralized  before  such  is  the  case.  With  such  indi- 
cators the  solution  must  be  kept  at  the  boiling  point  to  drive  off 
the  CO2. 


138  QUANTITATIVE  ANALYSIS 


SAMPLE   CALCULATION 

Weight  of  soda  ash  taken  =  4.9706. 

Solution  made  up  to  250  c.c.;  one-fifth  of  this,  50  c.c.,  titrated. 
4.9706  -5-  5  =  0.9941  gram  weight  of  soda  ash  in  each  50  c.c. 

What  is  the  weight  of  the  gaseous  HC1  in  1  c.c.  of  exactly  •=- 

HC1? 

HC1  (36.46) 


N 
How  much  NaaCOs  is  equivalent  to  1  c.c.  of  exactly  -    HC1? 


+  2  HCl  =  2  NaCl  +  H20  +  C02 
106  :  72.92  =  x  :  0.01823 
x  =  0.0265  gram, 

or  0.0265  gram  of  NaaCOs  is  equivalent  to  1  c.c.  of  -^  HCl. 


TlTRATIONS 


39.92  c.c.  2.82  c.c. 

1  c.c.  of  ^  HCl  =  1  c.c.  of^NaOH 


39.92  -  2.82  =  37.10  c.c.  of     HCl  actually  used  to  neutralize  the 

z 


soda  ash. 


N 
37.10  -jr  HCl  is  equivalent  to 


37.10  X  0.0265  =  0.98315  gram  of  Na2C03. 
What  per  cent,  is  0.98315  of  0.9941? 
0.98315 


0.9941 


=  98.89  per  cent  total  alkali. 


VOLUMETRIC  ANALYSIS  139 


THE  VOLUMETRIC  DETERMINATION  OF  CHLORIN 

IN  AN 
UNKNOWN  SALT 

A  Typical  Precipitation  Process 

The  salt  is: 

(a)  dissolved  in  water; 

(6)  titrated  with  approximately  yrr  AgNOa  with   potassium 

chromate  as  an  indicator; 
(c)  calculated  as  per  cent,  of  chlorin. 


REACTIONS 


I.  MCI  +  AgN03  =  AgCl  +  MNO3.  ~ 

II.  K2Cr04  +  2AgN03  =  Ag2Cr04  +  2  KN03. 
Red 


140  QUANTITATIVE  ANALYSIS 


THE  DETERMINATION  OF  CHLORIN 
PROCEDURE 

Weigh  into  a  six-inch  evaporating  dish  or,  if  the  titration  is  to 
be  done  on  a  white  surface,  into  a  350  c.c.  beaker  if  pre- 
ferred, two  portions  of  the  unknown  salt   weighing  the 
of  about  0.3  gram  each.  char*e 

Receive  from  the  instructor  about  250  c.c.  of  approximately 
YJT  silver  nitrate  solution!, —        Jr  c  — J  which  must  be  poured 

into  a  perfectly  clean  but  not  necessarily  dry  bottle.  This 
solution  must  be  kept  closely  stoppered  and  as  much  away 
from  the  light  as  possible. 


VOLUMETRIC    ANALYSIS  141 


N 
STANDARDIZATION  OF    THE    APPROXIMATELY     —    SOLUTION  OP 

SILVER  NITRATE  TO  DETERMINE  ITS  STRENGTH  IN 
TERMS  OF  CHLORIN 


Weigh  about  0.3  gram  of  pure  fused  sodium  weighing  the 
chlorid  (as  a  standard).  standard  salt 

Dissolve  in  cold  water.103 

N 
Titrate  with  the  approximately  -^.silver  nitrate  solution  using  as 

an  indicator  potassium  chromate  free  from   Titration  for  stand_ 

chlorin.104  *  ardization  of  the 

Calculate 

(1)  the  value  of  1  o*e«  of  the  AgNOs  in  terms  of  NaCl, 

(2)  the  per  cent,  of  Cl  in  NaCl  and 

(3)  the  value  of  1  c.c.  of  this  solution  of  AgNOs  in  terms 
of  chlorin. 

In  an  exactly    ^    solution   of  AgNO3,  1  c.c.   corresponds  to 
0.003545  gram  of  Cl. 


EXPLANATORY  FACTS 

103.  The  solution  of  the  salt  must  be  practically  neutral,  as 
acids  exercise  a  solvent  action  on  Ag2CrO4.     If  present,   acids 
must  be  neutralized  with  "  chlorin-f ree "  sodium  carbonate.     An 
excess  of  alkali,  however,  is  to  be  avoided,  as  it  will  cause  a  pre- 
cipitation of  Ag2C03. 

104.  As  long  as  any  chlorin  remains  unprecipitated,  no  red 
color  forms.     The  permanent  formation  of  the  red  A&CrO*  indi- 
cates the  end  point. 

*  "  Chemistry  of  the  Metals,"  Experiments  Nos.  51  and  54. 


142  QUANTITATIVE  ANALYSIS 


PROCEDURE 


Titrate  the  unknown  salt  just  as  the  pure 
sodium  chlorid  was  titrated,  using  K2Cr04 
as  an  indicator. 


VOLUMETRIC  ANALYSIS  143 


SAMPLE   CALCULATIONS 


Standardization  of  AgN03  Solution 


1.  Weight  of  pure  NaCl  taken  =  0.2603  gram. 

N 
Approximately      AgNO3  used  =  44.70  c.c. 


=  0.005823; 


that  is,  1  c.c.  of  the  AgNO3  solution  =  0.005823  gram  of  NaCl. 

2.  The  per  cent,  of  chlorin  in  NaCl  =  60.65. 

(The  student  is  to  show  by  calculation  in  the  notebook  that 
this  is  so.) 

3.  60.65  per  cent,  of  0.005823  =  0.003532. 
Therefore,  1  c.c.  of  AgNO3  =  0.003532  gram  of  Cl. 


DETERMINATION 

Weight  of  unknown  salt  used  =  0.3064  grams. 
Volume  of  AgNO3  used  =  42.60  c.c. 

0.003532  X  42.60 


0.3064 


=  49.10  per  cent,  chlorin. 


144  QUANTITATIVE  ANALYSIS 


THE  VOLUMETRIC  DETERMINATION  OF  IRON 

IN 
AN  ORE 

Iron  is: 

(a)  converted  to  ferric  chlorid,  FeCl3,  by  dissolving  the  ore 
in  hydrochloric  acid; 

(6)  changed  to  ferric  sulfate,  Fe^SO^s,  by  evaporation  with 
concentrated  sulfuric  acid; 

(c)  reduced  to  ferrous  sulfate,  FeS04,  by  zinc  and  by  hydrogen; 

N 

(d)  titrated  with  ^.  potassium  permanganate,  KMn04; 

(e)  calculated  as  per  cent,  of  iron. 


REACTIONS 

I.   FeaOs  +  6  HC1  =  2  FeCl3  +  3  H2O. 
II.   2  FeCl3  +  3  H2SO4  =  Fe»(SO4)8  +  6  HCl. 

III.  Zn  +  H2SO4  =  ZnSO4  +  H2. 

IV.  Fe2(SO4)3  +  Zn  =  ZnSO4  +  2  FeSO4. 

V.  Fe2(S04)3  +  H2  =  2  FeSO4  +  H2SO4. 

»>  I 

VI.   10  FeSO4  +  2  KMnO4  +  8  H2SO4 

=  5  FUN(SO4)3  +  I&04+  2  MnSO4  +  8  H2O. 
VII.   10  FeO  +  50 


VOLUMETRIC  ANALYSIS  145 


PREPARATION   OP   AN   JQ  SOLUTION    OP    POTASSIUM    PERMAN- 
GANATE, KMn04  (158.03) 


As  an  oxidizing  agent  in  acid  solution,  potassium  permanganate 
splits  up  as  follows: 

2  KMnO4  =  K2O.2  MnO.O5  (see  reactions  VI  and  VII). 

Each  two  molecules  of  KMnC>4  furnish  five  available  oxygen 
atoms,  which  are  equivalent  to  ten  hydrogen  atoms.  One  molecule 
of  KMnC>4  therefore  is  equivalent  to  five  atoms  of  hydrogen. 

The  molecular  weight  of  KMnO4  =  158.03. 
This  expressed  in  grams  =  158.03  grams. 

A  normal  solution  of  potassium  permanganate  used  as  an  oxi- 
dizing agent  contains  in  one  liter  31.60  grams,  one-fifth  of  its 

01  «n 

molecular  weight  in  grams.     A  decinormal  solution  contains     ' 
or  3.16  grams  in  one  liter. 


PROCEDURE 


N  Making  the 

Prepare  half  a  liter  of  an  y^  solution  of  potas-  standard  solution 

sium  permanganate. 
Warm  slightly  to  aid  solution. 
Filter  through  asbestos  on  a  Witt  plate. 


146  QUANTITATIVE  ANALYSIS 


STANDARDIZATION  OF  THE  -    KMnO4  SOLUTION  105 


N 
Rather  than  make  an  exact  JQ  KMnO4  solution,  it  is  usual 

to  standardize  it  in  terms  of  iron  or,  if  it  is  more  convenient,  in 
terms  of  any  other  appropriate  substance. 

Use  either  iron  wire  or  Mohr's  salt,  FeSO4.  (NH4)2SO4.6  H2O. 
If  iron  wire  is  used,  it  must  be  free  from  rust  and   Method  of  stand- 
be  of  a  known  percentage  purity.     If  Mohr's  salt 
is  used,  it  must  be  in  clear  crystals  (those  that  have   solution 
lost  no  water  of  crystallization)  or  those  from  a  sample,  the  iron 
content  of  which  has  been  gravimetrically  determined. 


PROCEDURE 

Weigh  into  two  Erlenmeyer  flasks  two  samples  of  iron  wire  of 

about  0.25  gram  each. 

Pour  into  each  flask  100  c.c.  of  dilute  sulfuric  add  (1.5). 
Loosely  cover  each  flask  with  a  one-inch  watch  glass  and  warm 

gently  till  all  the  iron  is  dissolved,  but  do  not  boil.     A  few 

flakes  of  carbon,  which  can  easily  be  dis-   standardization  of 

tinguished  as  such,  will  remain. 


EXPLANATORY   FACT 


105.  Solutions  of  KMnO4  undergo  decomposition  in  the  light. 
If  they  are  to  be  preserved  they  must  be  kept  either  in  colored 
glass  bottles  or  in  bottles  covered  with  black  paper.  Even  then, 
frequent  standardization  is  desirable. 


VOLUMETRIC   ANALYSIS  147 

After  the  foregoing,  to  be  certain  that  all  the  iron  is  in  the  ferrous 
state,  proceed  as  follows:  Get  ready  two  glass  tubes,  bent 
at  an  angle  of  about  45°,  with   the  longer  limb  about 
twelve  inches  and  the  shorter  about  three  inches  long.     In- 
sert the  short  limb  into  a  one-hole  rubber    Reduction  of  ^ 
stopper  of  proper  size  for  the  neck  of  the   oxidized  iron  to  the 
flask.     Support  the  two  flasks  with  clamps  ferrous  state 
on  ring  stands,  inclining  them  at  an  angle  of  45°.     When 
the  short  limb  is  inserted  into  the  neck  of  the  flask,  the  long 
limb  of  the  glass  tube  should  be  vertical. 

Put  into  each  flask  a  very  small  amount  of  solid  sodium  car- 
bonate,106 Na2CO3. 

Immediately  add  to  each  flask  a  very  small  amount  of  "40- 
mesh,"  chemically  pure  granular  zinc,107  and  stopper  the 
flask  at  once.* 

Dip  the  long  end  of  the  tubes  into  solutions   cooling  the  solution 
of   sodium  carbonate  contained  in  small   in  a  nommdizing 

beakers."®  atmosphere 

Gently  warm,  but  do  not  boil,  the  solution  in  the  inclined  flask 
till  the  zinc  is  dissolved.109 


EXPLANATORY  FACTS 

106.  The  carbon  dioxide  thus  evolved  will  expel  air  from  the 
flasks. 

107.  The  introduction  of  the  zinc  will  cause  the  reduction  to 
the  ferrous  state  of  any  iron  which  might  possibly  have  been 
oxidized  during  solution  (see  reactions  IV  and  V).     The  presence 
of  zinc  sulfate  hi  the  solution  (see  reaction  III)  does  not  affect 
the  titration  with  potassium  permanganate. 

108.  Although  the  hydrogen  can  escape,  yet,  since  the  end  of 
the  long  tube  is  sealed  by  the  sodium  carbonate  solution,  any 
ingress  of  air  is  impossible. 

109.  As  the  zinc  dissolves,  the  inclined  position  of  the  flask 
prevents  loss  by  spattering. 

*  "Chemistry  of  the  Metals,"  Experiment  No.  156. 


148  QUANTITATIVE  ANALYSIS 

Allow  the  flask  to  become  cold.110 

As  soon  as  the  solution  is  quite  cold,  it  should  be  at  once  titrated 
to  a  faint  pink  color  with  the  potassium  per-   Titration  of  the 
manganate  solution.111*112  ferrous  suifate 


EXPLANATORY  FACTS 

110.  As  the  solution  in  the  flask  cools,  the  sodium  carbonate 
solution  in  the  beaker  rises  in  the  tube,  but  the  first  drop  coming 
into  the  flask  causes  effervescence  and  the  liquid  is  driven  back 
down  the  tube.     This  gradual  equalization  of  pressure  continues 
until  equilibrium  is  established. 

111.  All  solutions  titrated  with  permanganate  of  potassium  must 
be  acid  —  preferably  with  sulfuric  acid  —  in  order  to  keep  the 
manganous  oxid  in  solution. 

112.  It  is  often  convenient  to  have  on  hand  a  solution  of 
Mohr's  salt  a  sample  of  which  has  been  titrated  with  the  KMnO4 
solution  so  that  the  value  of  one  cubic  centimeter  of  the  Mohr's 
salt  solution  in  terms  of  the  permanganate  solution  may  have 
been  previously  established.      If  the  end  point  should  ever  be 
exceeded,  the  solution  may  then  be  "titrated  back"  with  the 
Mohr's  salt  solution  and  allowance  made  for  the  KMnO4  which 
by  mistake  was  added  in  excess. 


VOLUMETRIC  ANALYSIS  149 


SAMPLE   CALCULATION 

Weight  of  iron  =  0.2500  gram  (99.85  per  cent.  pure). 

N 

y^KMnO4  solution  used  =  31.18  c.c. 

°-250031*80-9985°  0.0080.    Thatis' 
1  c.c.  of  the  KMnC>4  solution  is  equivalent  to  0.0080  gram  of  iron. 


150  QUANTITATIVE  ANALYSIS 


DETERMINATION  OF  THE  IRON 
Preparation  of  the  Ore  for  Analysis 

It  is  impossible  to  dissolve  and  therefore  to  analyze  an  ore  of 
this  nature  that  has  not  been  ground  to  an  impalpable  powder  too 
smooth  to  grit  between  the  teeth.     This  is  often  a  long  and  tedious 
operation  and  should  have  been  begun  before  the  student  has 
reached  this  determination  (see  page  122) .     Mortars  cut  from  agate 
(a  form  of  quartz)  must  be  used.    Do  not  put  much   Preparation  of  the 
more  of  the  ore  in  the  mortar  at  one  time  than  can   ore  by  Blindiae 
be  held  on  the  tip  of  a  small  spatula.     When  ground  to  an  impal- 
pable powder,  transfer  to  a  weighing  tube  and  grind  other  portions 
until  at  least  a  gram  of  the  ore  is  ready. 


PROCEDURE 

Weigh  out  into  porcelain  crucibles  two  portions  of  the  ground 
ore  of  about  0.5  gram  each. 

Heat  these  crucibles  in  the  Bunsen  flame  for  about  ten  min- 
utes.113 Roasting  the  ore 


EXPLANATORY   FACT 

113.  Iron  ores  often  contain  organic  matter,  which,  as  it  might 
be  later  acted  upon  by  the  permanganate  and  vitiate  the  results, 
must  be  destroyed  by  "roasting."  Moreover,  it  also  colors  the 
insoluble  residue  and  makes  it  difficult  to  tell  when  the  solution 
of  the  ore  is  complete. 


VOLUMETRIC  ANALYSIS  151 

Cool  the  crucibles  and  transfer  their  contents  to  two  250  c.c. 
casseroles.  If  all  the  ore  cannot  be  removed,  the  crucibles 
themselves  must  be  put  into  the  casseroles  as  well.114 

Add  to  each  sample  25  c.c.  of  hydrochloric  add  (1.2  sp.  gr.). 

Cover  the  casseroles  and  let  the  ore  digest  at  a  temperature  just 
below  boiling115  for  from  thirty  minutes  to  an  hour.     This 
will  generally  effect  complete  decomposi-  Preparation  of  the 
tion  as  shown  by  a  white  residue.     If  after  solution 
this  time  the  solvent  action  appears  to  have  ceased  and  the 
residue  is  still  dark,  proceed  as  follows: 

Dilute  the  solution,  filter  off  and  wash  the  residue  till  the  wash- 
ings give  no  test  for  acid. 

Ignite  the  filter  paper  and  residue  in  a  platinum  crucible  and 
fuse116  the  ash  with  a  small  quantity  of  Fusion  of  any 
sodium  carbonate,  Na^COa.  insoluble  residue 

After  cooling,  partly  fill  the  crucible  with  water  and  cautiously 
boil. 


EXPLANATORY  FACTS 

114.  Platinum  crucibles  must  not  be  put  into  iron  solutions. 

115.  The  iron  oxid  in  most  iron  ores,  if  ground  to  an  impalpable 
powder,  is  wholly  soluble  in  strong  hydrochloric  acid.     Hard  or 
prolonged  boiling  or  too  great  concentration  of  the  solution  of 
FeCls  must  be  avoided  to  prevent  loss  of  iron.     In  the  laboratories 
of  some  steel  works,  it  is  customary  to  leave  the  ore  in  the  hydro- 
chloric acid  on  steam  baths  over  night.     The  next  day  the  solu- 
tion is  usually  complete.     Some  analysts  advocate  adding  a  few 
drops  of  nitric  acid  to  hasten  the  solution. 

116.  By  fusion,  the  insoluble  substances  are  transformed  into 
such  compounds   as    silicates  of   the  alkalies,  carbonates  of  the 
heavy  metals,  etc.     This  fused  mass  is  entirely  decomposed  by 
treatment  with  dilute  hydrochloric   acid.      Therefore,  any   iron 
which  may  have  remained  undissolved  by  the  original  treatment 
with  acid  is  now  obtained  hi  the  solution  as  FeCl3. 


152  QUANTITATIVE  ANALYSIS 

Add  this  solution  and  any  residue  to  the  original  hydrochloric 
acid  solution. 

Carefully  wash  out  the  platinum  crucible  and   solution  of  the 
add  the  washings  to  the  solution.  fusion 

Warm  until  effervescence  ceases. 

Whether  or  not  this  secondary  process  was  needed,  a  white 
residue  need  not  be  filtered  off. 

Add  about  5  c.c.  of  concentrated  sulfuric  acid  to   conversion  to  the 
the  cooled  hydrochloric  acid  solution.  suUate 

Evaporate  under  the  hood  until  fumes  of  80s  are  evolved.117 

Cool  the  solution  and  dilute  to  about  100  c.c. 

Warm  the  liquid  until  nothing  but  flocculent  silica  remains 
undissolved. 

Transfer  quantitatively  to  a  250  c.c.  flask. 

Add  5  grams  of  "40-mesh"  C.  P.  granular  zinc  and  put  rubber 
stoppers  carrying  the  bent  glass  tubes,  as   Reduction  of  the 
described  on  page  147,  into  the  flasks.     The  iron 
tubes  should,  as  before,  dip  into  the  sodium  carbonate  solu- 
tion.    Avoid  hydrogen  explosions! 


EXPLANATORY  FACT 

117.  Potassium  permanganate  in  the  presence  of  hydrochloric 
acid  causes  the  oxidation  of  ferrous  salts,  as  will  be  seen  by  the 
following: 

2  KMnO4  +  10  FeCl2  +  16  HC1 

=  2  MnCl2  +  2  KC1  +  10  FeCl3  +  8H2O; 

but  it  is  also  true  that 

2  KMn04  +  16  HC1  =  2  KC1  +  2  MnCl2  +  5  C12  +  8  H2O. 

The  titration  with  permanganate,  then,  in  the  presence  of  hydro- 
chloric acid,  is  attended  with  a  possibility  of  loss  unless  special 
precaution,  such  as  adding  MnSO4,  etc.,  is  taken.  It  is  therefore 
customary  to  remove  all  the  hydrochloric  acid  by  means  of  this 
evaporation  with  sulfuric  acid  (see  reaction  II). 


VOLUMETRIC  ANALYSIS  153 

Allow  the  zinc  to  dissolve  entirely.     Aid  the  process  at  the  last 

by  a  gentle  heat.     Do  not  boil. 
With  the  tubes  still  sealed  by  the  sodium  carbonate  solution, 

allow  the  solutions  to  cool.     The  action   cooiingthe 

will  be  the  same  as  described  on  page  148.    solution 
When  cool,118  the  solution  should  be  at  once   xitration  of  the 

titrated   with   the   standardized    perman-   5nm 

ganate  solution. 


EXPLANATORY  FACT 

118.  The  complete  reduction  may  be  tested  for  as  follows:  With- 
draw a  minute  drop  of  the  solution  on  the  end  of  a  "drawn  out" 
glass  rod  and  touch  it  to  a  drop  of  KSCN  solution  on  the  white 
tile  of  the  buret  stand.  If  no  red  color  forms,  the  reduction  is 
complete.  If  a  red  color  does  appear,  more  zinc  must  be  added 
and  the  process  repeated. 


154  QUANTITATIVE  ANALYSIS 


SAMPLE   CALCULATION 

Weight  of  ore  used  =  0.5603  gram. 
KMnO4  solution  used  =  32.68  c.c. 

Standardization  of  KMnO4  solution  is  0.0080  of  gram  of  iron 
(see  page  149). 

32.68  X  0.0080  =  0.2614  gram  of  iron  "in  0.5306  gram  of  ore. 
0.2614 


0.5306 


=  49.26  per  cent,  of  iron. 


VOLUMETRIC   ANALYSIS 


155 


FACTORS  NEEDED  FOR  THE  DETERMINATIONS  IN  GRAVIMETRIC 
WORK  INCLUDED  IN  THIS  BOOK* 


Determina- 
tion of 

Weighed  as, 

Required. 

Factor. 

Log. 

Al 

A1203 

Al 

0.53033 

1.72455 

Cu 

CuO 

Cu 

0.79891 

1.90250 

Fe 

FeaOs 

Fe 

0.69944 

1.84475 

S04 

BaS04 

SO4 

0.41155 

1.61442 

Cl 

AgCl 

Cl 

0.24738 

1.39337 

MgO 

Mg2P207 

MgO 

0.  36219 

1.55894 

*  Other  factors,  if  needed,  may  be  found  in  Olsen's  Chemical  Annual,  in  the 
1  Chemiker  Kalender,"  or  Treadwell'a  "  Quantitative  Analysis." 


156 


QUANTITATIVE  ANALYSIS 


INTERNATIONAL  ATOMIC   WEIGHTS 


Aluminum   

Al 

0=16 
27.1 
120.2 
39.9 
75.0 
137.37 
208.0 
11.0 
79.92 
112.40 
132.81 
40.07 
12.00 
140.25 
35.46 
52.1 
58.97 
93.5 
63.57 
162.5 
167.7 
152.0 
19.0 
157.3 
69.9 
72.5 
9.1 
197.2 
4.0 
1.008 
114.8 
126.92 
193.1 
55.84 
82.9 
139.0 
207.1 
7.0 
174.0 
24.32 
54.93 
200.6 

Molybdenum  
Neodymium  
Neon  

...Mo 
...Nd 

...Ne 

0  =  16 

96.0 
144.3 
20.0 
58.68 
14.01 
190.9 
16.00 
106.' 
31.0 
195.0 
39.1 
140.6 
226.4 
102.9 
85.45 
101.7 
150.4 
44.1 
79.2 
28.3 
107.88 
23.00 
87.62 
32.07 
181.5 
127.5 
159.2 
204.0 
232.42 
168.5 
119.0 
48.1 
184.0 
238.5 
51.06 
128.0 

172.0 
89.0 
65.37 
90.6 

Sb 

A 

.As 
.Ba 
Ri 

Nickel  

...Ni 

B    " 

Nitrogen  

...N 

Bismuth     

Osmium  
Oxygen  
Palladium  

...Os 
...O 
...Pd 

Boron      

B 

Rr 

Cd 

Phosphorus  

...P 

Cs 

Platinum  

.  ...Pt 

Calcium         

.Ca 
.C 
Ce 

Potassium  
Praseodymium  .  .  . 
Radium  

...K 
...Pr 
..  .Ra 

Carbon  

Chlorin  

.Cl 
.Cr 
.Co 
.Cb 
Cn 

Rhodium  
Rubidium  
Ruthenium  
Samarium  

...Rh 
...Rb 
.  ..Ru 

...Sa 

Chromium  
Cobalt                .... 

Columbium  

Scandium  

...Sc 
Se 

Dysprosium  
Erbium 

•Dy 
F,r 

Selenium            .    . 

Silicon  
Silver  
Sodium 

...Si 
...Ag 

Na 

Europium  
Fluorin  

.Eu 
.F 
Gd 

Strontium  

...Sr 

.Ga 
.Ge 
.Gl 
An 

Sulphur  
Tantalum  

...S 
...Ta 

Tellurium 

Te 

Gold      

Terbium  
Thallium  
Thorium  
Thulium  

...Tb 

...Tl 
...Th 
...Tm 

Helium  

.He 
.H 
.In 
.1 
Tr 

Hydrogen  

lodin  
Indium  

Tin 

...Sn 
...Ti 
...W 
...U 

Titanium  
Tungsten  
Uranium  

Fe 

Kr 

Lanthanum  
Lead  

.La 
.Pb 
T,i 

Vanadium 

v 

Xenon 

...Xe 
ter- 
...Yb 

Ytterbium     (Neoyl 
bium)  

.Lu 
•  Mg 
.Mn 
Hg 

Magnesium  
Manganese  
Mercury  

Yttrium 

...Y 
...Zn 
...Zr 

Zinc  
Zirconium  

LOGARITHMS 


157 


LOGARITHMS 


-1 

Proportional  Parts. 

11 

0 

1 

- 

133 

456 

8   9 

10 

0000 

3043 

0086 

0128 

170 

212 

0253 

3294 

0334 

0374 

4    8  12 

7  21  25 

9  33  37 

11 

0414 

453 

0492 

0531 

569 

607 

0645 

3682 

0719 

0755 

4    8  11 

5  19  23 

6  30  34 

12 

0792 

828  0864 

0899 

0934 

969 

1004 

1038 

1072 

1106 

3    7  10 

4  17  21 

4  28  31 

13 

139 

173  1206 

1239 

271 

303  1335 

1367 

1399 

1430 

3    6  10 

3  16  19 

3  26  29 

14 

1461 

492  1523 

1553 

584 

614 

1644 

1673 

1703 

1732 

369 

2  15  18 

&  24  27 

15 

1761 

790   818 

1847 

875 

9031931 

1959 

1987 

2014 

368 

1  14  17 

20  22  25 

16 

2041 

01  is 

2095 

2122 

2148 

1752201 

2227 

2253 

2279 

358 

1  13  16 

8  21  24 

17 

2304 

:;:;n 

2355 

2380 

2405 

430:2455 

2480 

2504 

2529 

2         7 

0  12  15 

7  20  22 

18 

2553 

577 

2601 

2625 

2648 

672:2695 

2718 

2742 

2765 

2         7 

9  12  14 

6  19  21 

19 

2788 

2810 

2833 

2856 

2878 

900 

2923 

2945 

2967 

2989 

2         7 

9  11  13 

6  18  20 

20 

3010 

3032 

3054 

3075 

3096 

118 

3139 

3160 

3181 

3201 

2         6 

8  11  13 

5  17  19 

21 

3222 

3243 

3263 

3284 

3304 

32413345 

3365 

3385 

3404 

2          6 

8  10  12 

4  16  18 

22 

3424 

3444 

3464 

3483 

3502 

522  3541 

3560 

3579 

;:,<;>s 

2          6 

8  10  12 

4  15  17 

23 

3617 

3636 

3655 

3674 

3692 

711  3729 

3747 

3766 

3784 

2          6 

7    9  11 

3  15  17 

24 

3802 

3820 

3838 

3856 

3874 

892  3909 

3927 

3945 

3962 

2          5 

7    9  11 

12  14  16 

25 

3979 

3997 

4014 

4031 

4048 

4065  4082 

4099 

4116 

4133 

235 

7    9  10 

12  14  15 

26 

4150 

4166 

4183 

4200 

4216 

4232  '4249 

4265 

4281 

4298 

235 

7    8  10 

11  13  15 

27 

4314 

4330 

4346 

4362 

4378 

4393  [4409 

4425 

4440 

4456 

235 

689 

11  13  14 

28 

4472 

4487 

4502 

4518 

4533 

4548  4564 

4579 

4594 

4609 

2    3    5 

689 

11  12  14 

29 

4624 

4639 

4654 

4669 

4683 

4698  4713 

4728 

4742 

4757 

1    3    4 

679 

10  12  13 

30 

4771 

4786 

4800 

4814 

4829 

4843  4857 

4871 

4886 

4900 

1    3    4 

6          9 

10  11  13 

31 

4914 

4928 

4942 

4955 

4969 

4983  4997 

5011 

5024 

5038 

1    3    4 

6          8 

10  11  12 

32 

5051 

5065 

5079 

5092 

5105 

5119'5132 

5145 

5159 

5172 

1    3    4 

5         8 

9  11  12 

33 

5185 

5198 

5211 

5224 

5237 

52505263 

5276 

5289 

5302 

1    3    4 

5         8 

9  10  12 

34 

5315 

5328 

5340 

5353 

5366 

5378.5391 

5403 

5416 

5428 

1    3    4 

5         8 

9  10  11 

35 

5441 

5453 

5465 

5478 

5490 

5502  5514 

5527 

5539 

5551 

1    2    4 

5         7 

9  10  11 

36 

5563 

5575 

5587 

5599 

5611 

5623 

563" 

5647 

5658 

5670 

1    2    4 

5         7 

8  10  11 

37 

5682 

5694 

5705 

5717 

5729 

5740  5752 

5763 

5775 

5786 

1    2    3 

5         7 

8    9  10 

38 

5798 

5809 

5821 

5832 

5843 

5855  5866 

5877  5888 

5899 

1    2    3 

5          7 

8    9  10 

39 

5911 

5922 

5933 

5944 

5955 

5966  5977 

5988 

5999 

6010 

1    2    3 

457 

8    9  10 

40 

6021 

6031 

6042 

6053 

6064 

60756085 

6096  6107 

6117 

1    2    3 

5    6 

8    9  10 

41 

6128 

6138 

6149 

6160 

6170 

6180,6191 

6201  6212 

6222 

1    2    3 

5    6 

7    8    9 

42 

6232 

6243 

6253 

6263 

6274 

628416294 

6304  6314 

6325 

1    2    3 

5    6 

7    8 

43 

6335 

6345 

6355 

6365 

6375 

6385  6395 

64056415 

6425 

1    2    3 

5    6 

7    8 

44 

6435 

6444 

6454 

6464 

6474 

6484  6493 

65036513 

6522 

1    2    3 

5    6 

7    8 

45 

6532 

6542 

6551 

6561 

657 

6580  6590 

65996609 

6618 

1    2    3 

5    6 

7    8 

46 

6628 

6637 

6646 

6656 

6660 

66756684 

6693  6702 

6712 

1    2    3 

5    6 

7    7 

47 

6721 

6730 

6739 

6749 

6758 

6767 

6771 

6785  6794 

6803 

1    2    3 

5    5 

6    7 

48 

6812 

6821 

6830 

6839 

6848 

6857  6866 

6875 

(iSS 

6893 

1    2    3 

4    5 

678 

49 

6902 

6911 

6920 

6928 

693 

6946  6955 

6964  6972 

698 

1    2    3 

4    5 

678 

~50~ 

6990 

6998 

7007 

7016 

702 

7033 

7042 

7050705E 

7067 

1    2    3 

3         5 

678 

51 

52 

7076 
7160 

7084 
7168 

7093 
7177 

710 

7185 

711 

719 

7118 
7205 

7126 

7210 

713c 

7218 

7143 
7226 

7152 

7235 

1     2    3 
122 

3         5 
3          5 

678 

677 

53 

7243 

7251 

725S 

7267 

727 

7284 

[7292 

730C 

7308 

731 

1    2    2 

3          5 

667 

54 

7324 

7332 

734C 

7348 

735 

736^ 

[7372 

738C 

17388 

739 

1    2    2 

3          5 

667 

158 


QUANTITATIVE  ANALYSIS 


LOGARITHMS  —  Continued 


-J 
II 

55 
56 
57 

58 
59 

60 
61 
62 
63 
64 

~65~ 
66 
67 

68 
69 

70 
71 

72 
73 
74 

~75~ 
76 

77 
78 
79 

80 
81 
82 
83 

84 

0 

1 

7412 
7490 
7566 
7642 
7716 

7789 
7860 
7931 
8000 
8069 

It 

3 

4 

5 

6 

7451 
7528 
7604 
7679 
7752 

7825 
7896 
7966 
3035 
3102 

3169 
3235 
8299 
8363 
8426 

8488 
8549 
8609 

siifiO 
8727 

8785 
8842 
8899 
8954 
9009 

9063 
9117 
9170 
9222 
9274 

7 

7459 
7536 
7612 
7686 
7760 

7832 
7903 
7973 
8041 
8109 

8176 
8241 
8306 
8370 
8432 

8494 
8555 
8615 
8675 
8733 

8 

7466 
7543 
7619 
7694 
7767 

7839 
7910 
7980 
8048 
8116 

g 

Proportional  Parts. 

133 

456 

789 

"404 
7482 
7559 
7634 
7/09 

7782 
7853 
7924 
7993 
8062 

7419 

7497 
7574 
7649 
7723 

7796 
7868 
7938 
8007 
8075 

7427 
7505 
7582 
7657 
7731 

7803 

7875 
7945 
8014 
8082 

7435 
7513 
7589 
7664 

7738 

7810 
7882 
7952 
8021 
8089 

7443 
7520 
7597 
7672 
7745 

818 
889 
959 
028 
096 

7474 
7551 
7627 
7701 

7774 

7846 
7917 
7987 
8055 
8122 

2 
2 
2 

2 
2 
2 
2 
2 

3  5 
3  5 
3  5 
3  4 
3  4 

3  4 
3  4 
334 
334 
334 

5  6  7 
5  6  7 
567 
5  6  7 
567 

566 
6  6 
6  6 
5  6 
5  6 

8129 
8195 
8261 
8325 
8388 

8451 
8513 

fel 

8692 

8136 
8202 
8267 
8331 
8395 

8457 
8519 

8579 
8639 
8698 

8142 
8209 
8274 
8338 
8401 

8463 
8525 
8585 
8645 
8704 

8149 
8215 
8280 
8344 
8407 

8470 
8531 
8591 
8651 
8710 

8156 

8222 
8287 
8351 
8414 

8476 
8537 
8597 
8657 
8716 

162 

8228 
8293 
8357 
8420 

8482 
8543 
8603 
8663 

8722 

8182 
8248 
8312 
8376 
8439 

8500 
8561 
8621 
8681 
8739 

8189 
8254 
8319 
8382 
8445 

8506 

s,-,ii7 
8627 
8686 
8745 

2 
2 
2 
2 
2 

2 

2 
2 

334 
334 
334 
334 
234 

234 
234 
234 
234 
234 

5 
5 

fi 
5 
5 
5 
5 

8751 
8808 
8865 
8921 
8976 

9031 
9085 
9138 
9191 
9243 

8756 
8814 
8871 
8927 
8982 

9036 

9090 
9143 
9196 
9248 

9299 
9350 
9400 
9450 
9499 

9547 
9595 
9643 
9689 
9736 

9782 
9827 
9872 
9917 
9961 

8762 
8820 
8876 
8932 
8987 

9042 
9096 
9149 
9201 
9253 

8768 
8825 
8882 
8938 
8993 

9047 
9101 
9154 
9206 
9258 

8774 
8831 
8887 
8943 
8998 

9053 
9106 
9159 
9212 
9263 

8779 
8837 
8893 
8949 
9004 

9058 
9112 
9165 
9217 
9269 

8791 
8848 
8904 
8960 
9015 

9069 
9122 
9175 
9227 
9279 

8797 
8854 
8910 
8965 
9020 

9074 
9128 
9180 
9232 
9284 

9335 
9385 
9435 
9484 
9533 

9581 
9628 
9675 
9722 
9768 

8802 
8859 
8915 
8971 
9025 

9079 
9133 
9186 
9238 
9289 

2 
2 
2 
2 
2 

2 
2 
2 
2 
2 

233 
233 
233 
233 
233 

233 
233 

233 
2  3  3 

5 
5 
5 
5 
5 

5 
5 
5 
5 
5 

85 
86 
87 
88 
89 

90 
91 
92 
93 
94 

9294 
9345 
9395 
9445 
9494 

9542 
9590 
9638 
9685 
9731 

9304 
9355 
9405 
9455 
9504 

9552 
9600 
9647 
9694 
9741 

9786 
9832 
9877 
9921 
996£ 

9309 
9360 
9410 
9460 
9509 

9557 
9605 
9652 
9699 
9745 

9315 
9365 
9415 
9465 
9513 

9562 
9609 
9657 
9703 
9750 

9320 
9370 
9420 
9469 
9518 

9566 
9614 
9661 
9708 
9754 

9325 
9375 
9425 
9474 
9523 

9571 
9619 
9666 
9713 
9759 

9330 
9380 
9430 
9479 
9528 

9576 
9624 
9671 
9717 
9763 

9340 
9390 
9440 
9489 
9538 

9586 
9633 
9680 
9727 
9773 

2 

2 
0 
0 
.0 

0 
0 
0 
0 
0 

233 
233 
223 
223 
223 

2  2 
2  2 
2  2 
2  2 
2  2 

4    5 
4    5 
3 
3 
3 

3 
3 
3 
3 
3 

95 
96 
97 
98 
99 

9777 
9823 
9868 
9912 
9956 

9791 
9836 
9881 
992C 
996£ 

9795 
9841 
9886 
9930 
9974 

9800 

9845 
9890 
9934 
9978 

9805 
9850 
9894 
9939 
9983 

9809 
9854 
9899 
9943 
9987 

9814 
9859 
9903 
9948 
9991 

9818 
9863 
9908 
9952 
9996 

0 
0 
0 
0 
0 

223 
223 
223 
223 
223 

3 
3 
3 
3  4 
3  3 

LOGARITHMS 


159 


LOGARITHMS.  —  Continued 


2 

aS 

SI 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  Parts. 

.00 
.01 
.02 
.03 
.04 

.05 

.06 
.07 
.08 
.09 

1000 
1023 
1047 
1072 
1096 

1122 
1148 
1175 
1202 
1230 
1259 
1288 
1318 
1349 
1380 

1413 
1445 
1479 
1514 
1549 
1585 
1622 
1660 
1698 
1738 

1778 
1820 
1862 
1905 
1950 

1002 
1026 
1050 
1074 
1099 

1125 
1151 
1178 
1205 
1233 
1262 
1291 
1321 
1352 
1384 

1416 
1449 
1483 
1517 
1552 
1589 
1626 
1663 
1702 
1742 

1782 
1824 
1866 
1910 
1954 

1005 
1028 
1052 
1076 
1102 

1127 
1153 
1180 
1208 
1236 
1265 
1294 
1324 
1355 
1387 

1419 
1452 
1486 
1521 
1556 
1592 
1629 
1667 
1706 
1746 

1786 
1828 
1871 
1914 
1959 

1007 
1030 
1054 
1079 
1104 

1130 
1156 
1183 
1211 
1239 
1268 
1297 
1327 
1358 
1390 

1422 
1455 
1489 
1524 
1560 

1009 
1033 
1057 
1081 
1107 

1132 
1159 
1186 
1213 
1242 
1271 
1300 
1330 
1361 
1393 

1426 
1459 
1493 
1528 
1563 
1600 
1637 
1675 
1714 
1754 

1795 
1837 
1879 
1923 
1968 

1012 
1035 
1059 
1084 
1109 

1135 
1161 
1189 
1216 
1245 
1274 
1303 
1334 
1365 
1396 

1429 
1462 
1496 
1531 
1567 
1603 
1641 
1679 
1718 
1758 

1799 
1841 
1884 
1928 
1972 

1014 
1038 
1062 
1086 
1112 

1138 
1164 
1191 
1219 
1247 

1016 
1040 
1064 
1089 
1114 

1140 
1167 
1194 
1222 
1250 

1019 
1042 
1067 
1091 
1117 

1143 
1169 
1197 
1225 
1253 
1282 
1312 
1343 
1374 
1406 

1439 
1472 
1507 
1542 
1578 
1614 
1652 
1690 
1730 
1770 

1811 
1854 
1897 
1941 
1986 

1021 
1045 
1069 
1094 
1119 

1146 
1172 
1199 
1227 
1256 
1285 
1315 
1346 
1377 
1409 

1442 
1476 
1510 
1545 
1581 
1618 
1656 
1694 
1734 
1774 

1816 
1858 
1901 
1945 
1991 

0  0 
0  0 
0  0 
0  0 
0  1 

0 
0 
0 
0 
0 

1 
1 
1 
1 
2 

2 
2 
2 
2 
2 

2  2 
2  2 
2  2 
2  2 
2  2 

222 
222 
222 
223 
223 

.10 
.11 
.12 
.13 
.14 

.15 

.16 
.17 
.18 
.19 

1276 
1306 
1337 
1368 
1400 

1432 
1466 
1500 
1535 
1570 
1607 
1644 
1683 
1722 
1762 

1803 
1845 
1888 
1932 
1977 

1279 
1309 
1340 
1371 
1403 

1435 
1469 
1503 
1538 
1574 
1611 
1648 
1687 
1726 
1766 

1807 
1849 
1892 
1936 
1982 

0 
0 
0 
0    1 
0    1 

0    1 
0 
0 
0 
0 

1  2 

2  2 
2  2 
2  2 

2  2 

2  2 
2  2 
2  2 

223 
223 
223 
233 
233 

233 
233 
233 
233 
333 

.20 
.21 
.22 
.23 
.24 

.25 

.26 
.27 
.28 
.29 

1596 
1633 
1671 
1710 
1750 

1791 

1832 
1875 
1919 
1963 

0 
0 
0 
0 
0 

0 
0 
0 
0 
0 

2  2 
222 
222 
222 

222 
223 
223 
223 
223 

333 
333 
333 
3  3 
3  3 

3 
3 
3 

.30 
.31 
.32 
.33 
.34 

.35 

.36 
.37 
.38 
.39 

1995 
2042 
2089 
2138 
2188 

2239 
2291 
2344 
2399 
2455 

2000 
2046 
2094 
2143 
2193 

2244 
2296 
2350 
2404 
2460 

2004  2009 
2051  2056 
2099  2104 
21482153 
21982203 

22492254 
23012307 
23552360 
241012415 
2466  2472 

2014 
2061 
2109 
2158 
2208 

2259 
2312 
2366 
2421 

2477 

2018 
2065 
2113 
2163 
2213 

2265 
2317 
2371 

2427 
2483 

2023 
2070 
2118 
2168 
2218 

2270 
2323 
2377 
2432 
2489 

2028 
2075 
2123 
2173 
2223 

2275 
2328 
2382 
2438 
2495 

2032 
2080 
2128 

2178 
2228 

2280 
2333 
2388 
2443 
2500 

2037 
2084 
2133 
2183 
2234 

2286 
2339 
2393 
2449 
2506 

0 
0 
0 
0 

1 

2 
2 

223 
223 
223 
223 
233 

2  3 
2  3 
2  3 
2  3 
2  3 

.40 
.41 
.42 
.43 
.44 

.45 
.46 
.47 
.48 
.49 

2512 
2570 
2630 
2692 
2754 

2818 
2884 
2951 
3020 
3090 

2518 
2576 
2636 
2698 
2761 

2825 
2891 
2958 
3027 
3097 

2523 
2582 
2642 
2704 
2767 

2831 

2897 
2965 
3034 
3105 

2529 
2588 
2649 
2710 
2773 

2838 
2904 
2972 
3041 
3112 

2535 
2594 
2655 
2716 
2780 

2844 
2911 
2979 
3048 
3119 

2541 
2600 
2661 
2723 
2786 

2851 
2917 
2985 
3055 
3126 

2547 
2606 
2667 
2729 
2793 

2858 
2924 
2992 
3062 
3133 

2553 
2612 
2673 
2735 
2799 

2864 
2931 
2999 
3069 
3141 

2559 
2618 
2679 
2742 
2805 

2871 
2938 
3006 
3076 
3148 

2564 
2624 
2685 
2748 
2812 

2877 
2944 
3013 
3083 
3155 

2 

2 
2 
2 
2 

2 
2 
2 
2 
2 

2  3 
2  3 
2  3 
3  3 
3  3 

3  3 
3  3 
3  3 
3  4 
3  4 

5 

5 
6 
6 
5  6 

556 
556 
556 
566 
566 

160 


QUANTITATIVE  ANALYSIS 


LOGARITHMS  —  Concluded 


A§ 

m 

0 

1 

2 

3 

4 

5 

6 

7 

8 

9 

Proportional  Parts. 

123 

456 

789 

.50 
.51 
.52 
.53 
.54 

.55 
.56 
57 
.58 
.59 

3162 
3236 
3311 

3388 
3467 

3548 
3631 
3715 
3802 
3890 

3170 
3243 
3319 
3396 
3475 

3556 
3639 
3724 
3811 
3899 
3990 
4083 
4178 
4276 
4375 

4477 
4581 
4688 
4797 
4909 

3177 
3251 
3327 
3404 
3483 

3565 
3648 
3733 
3819 
3908 

3184 
3258 
3334 
3412 
3491 

3573 
3656 
3741 
3828 
3917 

3192 
3266 
3342 
3420 
3499 

3581 
3664 
3750 
3837 
3926 

3199 
3273 
3350 

3428 
3508 

3589 
3673 
3758 
3846 

!!«() 

3206 
3281 
3357 
3436 
3516 

3597 
3681 
3767 
3855 
3945 

3214 
3289 
3365 
3443 
3524 

3606 
3690 
3776 
3864 
3954 

3221 
3296 
3373 
3451 
3532 

3614 
3698 
3784 
3873 
3963 

3228 
3304 
3381 
3459 
3540 

3622 
3707 
3793 
3882 
3972 

2    2 
2    2 
2    2 
2    2 

2    2 
2    3 

344 
345 
345 

567 

567 
567 

345 
345 

677 
678 

2    3 
2    3 

4    5 
5    5 

678 
678 

.60 
.61 
.62 
.63 
.64 

.65 
.66 
.67 
.68 
.69 

3981 
4074 
4169 
4266 
4365 

4467 
4571 
4677 
4786 
4898 

3999 
4093 
4188 
4285 
4385 

4487 
4592 
4699 
4808 
4920 

4009 
4102 
4198 
4295 
4395 

4498 
4603 
4710 
4819 
4932 

4018 
4111 
4207 
4305 
4406 

4508 
4613 
4721 
4831 
4943 

4027 
4121 
4217 
4315 
4416 

4519 

4624 
4732 
4842 
4955 

4036 
4130 
4227 
4325 
4426 

4529 
4634 
4742 
4853 
4966 

4046 
4140 
4236 
4335 
4436 

4539 
4645 
4753 
4864 
4977 
5093 
5212 
5333 
5458 
5585 

5715 
5848 
5984 
6124 
6266 

4055 
4150 
4246 
4345 
4446 

4550 
4656 
4764 
4875 
4989 
5105 
5224 
5346 
5470 
5598 

5728 
5861 
5998 
6138 
6281 

4064 
4159 
4256 
4355 

2    3 
2    3 
2    3 
2    3 

5    6 
5    6 
5    6 
456 

678 
789 
789 
789 

4560 
4667 
4775 
4887 
5000 
5117 
5236 
5358 
5483 
5610 

5741 
5875 
6012 
6152 
6295 

2    3 
2    3 
2    3 
2    3 

456 
456 
457 
467 
567 

7    8    9 
7    9  10 
8    9  10 
8    9  10 
8    9  10 

.70 
.71 
.72 
.73 
.74 

.75 

.76 
.77 
.78 
.79 

5012 
5129 
5248 
5370 
5495 

5623 
5754 
5888 
6026 
6166 

5023 
5140 
5260 
5383 
5508 

5636 
5768 
5902 
6039 
6180 

5035 
5152 
5272 
5395 
5521 

5649 
5781 
5916 
6053 
6194 

5047 
5164 
5284 
5408 
5534 

5662 
5794 
5929 
6067 
6209 

5058 
5176 
5297 
5420 
5546 

5675 
5808 
5943 
6081 
6223 

5070 
5188 
5309 
5433 
5559 

5689 
5821 
5957 
6095 
6237 

5082 
5200 
5321 
5445 
5572 

5702 
5834 
5970 
6109 
6252 

2    4 
2    4 
2    4 
3    4 
3    4 

3    4 
3    4 
3    4 
3    4 
3    4 

567 
567 
567 
568 
568 

578 
578 
578 
678 
679 

8    9  11 
8  10  11 
10  11 
10  11 
10  12 

10  12 
11  12 
10  11  12 
10  11  13 
10  11  13 

.80 
.81 
.82 
.83 
.84 

.85 
.86 
.87 
.88 
.89 
.90 
.91 
.92 
.93 
.94 

.95 
.96 
.97 
.98 
.99 

6310 
6457 
6607 
6761 
6918 

7079 
7244 
7413 
7586 
7762 
7943 
8128 
8318 
8511 
8710 

8913 
9120 
9333 
9550 

9772 

6324 
6471 
6622 
6776 
6934 

7096 
7261 
7430 
7603 
7780 

6339 
6486 
6637 
6792 
6950 

7112 

7278 
7447 
7621 
7798 
7980 
8166 
8356 
8551 
8750 

8954 
9162 
9376 
9594 
9817 

6353 
6501 
6653 
6808 
6966 

7129 
7295 
7464 
7638 
7816 
7998 
8185 
8375 
8570 
8770 

8974 
9183 
9397 
9616 
9840 

6368 
6516 
6668 
6823 
6982 

7145 
7311 
7482 
7656 
7834 
80l7 
8204 
8395 
8590 
8790 

8995 
9204 
9419 
9638 
9863 

6383 
6531 
6683 
6839 
6998 

7161 
7328 
7499 
7674 
7852 

6397 
6546 
6699 
6855 
7015 

7178 
7345 
7516 
7691 

7870 

6412 
6561 
6714 
6871 
7031 

7194 
7362 
7534 
7709 
7889 

6427 
6577 
6730 
6887 
7047 

7211 
7379 
7551 

7727 
7907 

6442 
6592 
6745 
6902 
7063 

7228 
7396 
7568 
7745 
7925 

134 
235 
235 
235 
235 

235 
235 
235 
2         5 
2         5 

679 
689 
689 
689 
6    8  10 

7    8  10 
7    8  10 
7    9  10 
7    9  11 
7    9  11 

10  12  13 
11  12  14 
11  12  14 
11  13  14 
11  13  15 

12  13  15 

12  13  15 
12  14  16 
12  14  16 
13  14  16 

7962 
8147 
8337 
8531 
8730 

8933 
9141 
9354 
9572 
9795 

8035 
8222 
8414 
8610 
8810 

9016 
9226 
9441 
9661 

fese 

8054 
8241 
8433 
8630 
8831 

9036 
9247 
9462 
9683 
9908 

8072 
8260 
8453 
8650 
8851 

9057 
9268 
9484 
9705 
9931 

8091 

8279 
8472 
8670 
8872 

9078 
9290 
9506 
9727 
9944 

8110 

8299 
8492 
8690 
8892 

9099 
9311 
9528 
9750 
9977 

2         6 
2         6 
2         6 
2          6 
2         6 

2         6 
2         6 
2          7 
2          7 
257 

7    9  11 
8    9  11 
8  10  12 
8  10  12 
8  10  12 

8  10  12 
8  11  13 
9  11  13 
9  11  13 
9  11  14 

13  15  17 
13  15  17 
14  15  17 
14  16  18 
14  16  18 

15  17  19 
15  17  19 
15  17  20 
16  18  20 
16  18  20 

INDEX 


Absolute  value  of  a  standard  hydrochloric  acid  solution,  determination  of.     121 

Acidimetry 112 

Aliquot  part 90,  136 

Alkalimetry 112 

Alkalinity,  total,  of  soda  ash,  determination  of 134 

Alloy  for  electrolytic  determination 89 

Aluminium,  determination  of,  in  alum 33 

removal  of,  from  dolomite 71 

Analysis,  gravimetric  and  volumetric,  compared 2,  97 

Anode 86 

Apparatus 5 

for  measuring  volume  of  liquids 98 


Balances 10 

adjustment  of 11 

Barium  sulfate,  precipitating  and  filtering  of 59 

Baths,  air  and  steam 7 

Burets f..  99 

reading  of 100 


Calcium,  determination  of,  in  a  mineral 73 

Calcium  oxalate,  purification  of 74 

Calculation 27 

Calibration  of  apparatus  for  measuring  volume 98 

Cathode  and  cation 86 

Checking  of  weights  by  instructor 16,  30 

Chlorin,  gravimetric  determination  of,  in  a  soluble  salt 62 

volumetric  determination  of,  in  a  soluble  salt 139 

161 


162  INDEX 


Cleaning  volumetric  glassware 101 

Constant  weight 26 

Coolers  for  reduced  iron  solutions 117 

Copper  in  an  alloy,  electrolytic  determination  of 92 

determination  of,  in  copper  sulfate 45 

sulfate,  preparation  of  pure  crystals  of *. . .  46 

oxid,  method  of  igniting 49 

Crucibles 7 

cleaning  of 8 

Gooch 25 

Current  density 86 


Decantation 23 

Desiccators 10 

use  of 41 

Dissociation 20,  85 

Dolomite  (or  similar  minerals),  analysis  of 68 


Electrodes 86 

drying  of 92 

Electrolysis 86 

Electrolyte 86 

Electrolytic  Analysis 85-94 

Electrolytic  determinations 87 

End  point,  determination  of 103 

Evaporations 7,  24 


Factors,  calculation  and  use  of 27,  155 

for  approximately  normal  solutions 119 

Filter  flasks 7 

Filtering 22 

Filter  paper 5,  22 

for  barium  sulfate 60 

tared 25 

Filtration,  arrangement  of  apparatus  for 36 

Funnels 6 

Fusion  of  insoluble  substances 17,  79 


Gibbs,  Wolcott,  method  of,  for  magnesium 76 

Glassware,  cleaning  of  surface  of,  in  volumetric  analysis 101 

Gooch  crucibles 25 

Gravimetric  Analysis 33-81 

definition  of .  .  12 


INDEX  163 


Hydrochloric  acid,  preparation  of  half-normal  solution  of 113 

determination  of  absolute  value  of  solution  of. 121 

dilution  of  solution  of,  to  an  exact  value 124,  127 


Iceland  spar,  use  of,  as  a  standard 122 

Ignition  of  precipitates 25 

Indicators 105 

Ions 20,  85 

Iron,  gravimetric  determination  of,  in  a  soluble  salt 53 

volumetric  determination  of,  in  a  soluble  salt 144 

and  SO4,  separation  of 55,  58 

removal  of,  from  dolomite 71 

Isolation  of  elements  or  radicals ...  17 


Lacmoid 107 

Liquid  level,  reading  of 99,  100 

Litmus 106 


Magnesium,  determination  of,  in  dolomite 76 

Magnesium  oxalate,  properties  of 74 

Magnesium  ammonium  phosphate,  ignition  of,  to  magnesium  pyrophos- 

phate 77 

Manganese,  removal  of,  from  dolomite 71 

Measuring  flasks 99 

Meniscus 99,  100 

Methyl  orange 106 

Microcosmic  salt,  use  of,  for  the  determination  of  magnesium 76 


Normal  solutions 108 

adjustment  of,  by  dilution  to  an  exact  volume 124,  127,    / 

128,  132    *• 
Notebooks.  .  .  30 


Permanency  of  standard  solutions Ill 

Phenolphthalein 107 

Pipets 98 

Platinum,  rules  for  use  of 8 

"Policeman,"  use  of,  for  removing  precipitates 5,  38 

Potassium  cyanid,  use  of 89,  90 

Potassium  permanganate,  preparation  and  standardization  of  a  tenth- 
normal  solution  of ...  145 


164  INDEX 

PAGE 

Precipitates,  drying  of 7,  39,  40 

ignition  of 25,  40 

purification  of 18 

washing  of 17,  20 

Precipitation 17,  20 

Qualitative  and  quantitative  analysis,  comparison  of 1 

Quills,  use  of,  for  removing  precipitates 38 

Radical  SO4,  gravimetric  determination  of,  in  a  soluble  salt 57 

Reagents 17 

amounts  of,  to  add. 18 

Records,  types  of 31,  43 

Relative  values  of  acid  and  alkali  solutions,  determination  of 116 

Separation  of  elements  or  radicals 17 

Silica,  determination  of,  in  minerals  and  rocks 78 

purification  of,  by  hydrofluoric  acid 81 

removal  of,  from  dolomite 70 

from  iron  ore 151 

Silicates,  fusion  of,  to  accomplish  solution 17,  79 

Silicic  acid,  dehydration  of 70,  80 

Silver,  electrolytic  determination  of 91 

Silver  chlorid,  action  of  light  upon 63,  66 

method  of  igniting 65 

Silver  nitrate,  standardization  of  a  solution  of 141,  143 

Soda  ash,  determination  of  total  alkalinity  of 134 

Sodium  hydroxid,  dilution  of  a  solution  of,  to  exact  value ....     128,  131,  132 

preparation  of  a  half-normal  solution  of 115 

standardization  of,  against  oxalic  acid 129 

Solubilities,  table  of 21 

Solution,  process  of 17 

Solutions,  facts  regarding 19 

homogeneity  of 19 

Solvents 19 

Standard  solutions 102 

permanency  of Ill,  115 

dilution  of,  to  exact  value 121,  125 

Study  outside  the  laboratory 32 

Suction  for  filtrations 7,  22 

Sulfuric  acid,  determination  of,  in  a  soluble  salt 57 

Tared  filter  papers 25 

Titration .  .  102 


INDEX  165 

PAGE 

Volumetric  Analysis 97-154 

definition  of 97 


Washing  of  precipitates 22,  37,  38 

test  for  the  completion  of 22 

Washing  by  decantation 23,  37 

Weighing,  directions  for 15 

precautions  for 13 

Weighing  tubes 6 

Weights,  analytical 11 

average  approximate 15 

constant 26 

Work,  planning  of 29 


Zero  point,  determination  of 14 

Zinc,  "40-mesh,"  use  of,  to  reduce  iron  in  volumetric  determination. . .     147 


Printed  in  the  United  States  of  America. 


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